The following articles are excerpted from the Vegetable Advisory Team Alert newsletters. Additional advice and updates are available at Dr. Mary Hausbeck's lab page:
http://plantpathology.msu.edu/labs/hausbeck/hausbeckVineCrops.htm

Map of confirmed downy mildew by counties.

To read additional issues of the newsletter, please visit: http://www.ipm.msu.edu/veg-cat.htm

Downy mildew confirmed on the west side of the state
Mary K. Hausbeck
Plant Pathology

Downy mildew
As of this morning, July 9, downy mildew has been found in Allegan County on pickling cucumbers. This is in addition to the earlier confirmation from Monroe County (see map). All cucumber growers must apply downy mildew fungicides immediately! See the tables below for spray recommendations. Finding downy mildew on the west side of the state is very important and likely means that the pathogen in now widespread in the state. Updates on the movement of downy mildew in Michigan and recommended fungicide programs can be found on the Internet at: (http://plantpathology.msu.edu/labs/hausbeck/ForGrowers.html).

Phytophthora crown, fruit and root rot
Phytophthora blight should be not forgotten, and will undoubtedly cause problems for processing growers as we reach harvesting. Pickle growers must remember the formula for protecting the fruit: Apply sprays when fruits are one inch, three inches, and five inches. Young fruit are especially susceptible and must be protected early. The fruit must be covered with the fungicide spray to be adequately protected. Covering only the foliage with fungicide is not good enough because the fungicide will not be absorbed by the foliage and moved into the fruit. This means that applying products by airplane is not considered the most effective means of protecting against phytophthora fruit rot.

Some fungicides offer control of both downy mildew and phytophthora. Last year in our phytophthora trials, Gavel and Presidio performed better than other products that were tested and are also products that protect against downy mildew. Since many pickle fields have phytophthora that is resistant to the Ridomil-type products, I hesitate to recommend them for use. Prior to fruit formation, I would recommend the downy mildew-only fungicides and once fruit have formed, move to products that provide protection against downy mildew and phytophthora.

Note: This article was originally made available Thursday, July 3 in the web edition of the Vegetable CAT Alert.

On July 2, a technician from my lab detected downy mildew on cucumbers growing in Monroe County. This field was selected for scouting because it is where the spore trap is running (see spore counts at end of this article). The infection is very light in this field and no fungicides had been applied. Based on the leaf samples, the infection is relatively recent. What I find to be worrisome is that all five spore traps in the state have now detected low levels of downy mildew spores in the air. With this field occurrence and recent storms, it is guaranteed that the spore counts will increase and outbreaks become more numerous. I find it hard to believe that this outbreak in Monroe County is the only one in the state. Therefore, it is crucial that cucumber growers in the state begin their downy mildew spray program immediately! For the processing pickle growers who may have only recently planted, it is important to remember that young seedlings readily become infected with downy mildew. Seedlings that have their cotyledons infected with downy mildew create a miserable situation and must be avoided. Banded fungicide sprays over very young plantings can save a significant amount of money.

Be assured that my program has tested hundreds of fungicide programs. The only fungicides that we can recommend are those listed in the two tables below. However, if the intervals are stretched or the rates lowered, these programs will not protect cucumbers against downy mildew as the disease intensifies in the state. If you are growing other sorts of cucurbits such as pumpkins, zucchinis, gourds, or squash, it is important to note that these crops are not as susceptible to downy mildew as cucumbers. Sprays for these crops can be delayed until downy mildew is noted in that particular county as long as they are being scouted frequently. In past years, melons have become infected with downy mildew and should be treated with a protective fungicide program. For melons, the program can be a bit less intensive than the program recommended for cucumbers.

• Fungicides should be applied to cucumbers every FIVE (5) to SEVEN (7) days.
  
• Fungicides should be applied to pumpkins, melons, cantaloupes, squash, zucchini and gourds every SEVEN (7) to TEN (10) days.

Downy mildew confirmed in Lenawee County
Mary Hausbeck
Plant Pathology

Downy mildew was confirmed on cucumber in a third Michigan county, Lenawee, on July 17 (see map). Fungicide recommendations continue to be as follows:

• Fungicides should be applied to cucumbers every FIVE to SEVEN days.
  
• Fungicides should be applied to pumpkins, melons, cantaloupes, squash, zucchini and gourds every SEVEN to TEN days.

Downy mildew confirmed in Gratiot County
Dr. Mary Hausbeck
Plant Pathology

Downy mildew was confirmed on cucumber in Gratiot County on July 9.  Fungicide recommendations are summarized here:

• Fungicides should be applied to cucumbers every FIVE (5) to SEVEN (7) days.
• Fungicides should be applied to pumpkins, melons, cantaloupes, squash, zucchini and gourds every SEVEN (7) to TEN (10) days.

How downy mildew spores are monitored:
Five spore traps were placed in different counties in Michigan on the following dates:

1. Monroe County: June 7
   
2. Allegan County: June 11
   
3. St. Joseph County: June 13
   
4. Bay County: June 14
   
5. Saginaw County: June 14
   

The reel inside the spore trap must be changed every seven days. At that time, a new reel is placed in the trap, and the reel with the spores is brought back to the lab where it is processed.  Processing of one spore trap reel takes approximately four hours. The tape on the reel is removed and placed onto a cutting block, where it is cut into seven pieces that represent 24 hours and each hour is marked. Each 24-hour section of tape is put onto a microscope slide and stained. A cover slip is applied, sealed to the slide and allowed to dry.  Once the slides are prepared, they are placed under the microscope and the entire tape is examined for downy mildew spores. Spores are counted and logged into a data sheet under the specific hour of the specific day that they were in the air around that particular spore trap. To count one reel of slides takes one to two days.

Table 2 summarizes the spore numbers that have been counted so far for 2007 in the highlighted columns.  For comparison purposes, spore counts from 2006 are in the adjacent, non-highlighted columns.  The entire table containing all of 2006 data is available at my website (http://plantpathology.msu.edu/labs/hausbeck/Hausbeck%20Lab%20For%20Growers.html).  This table is continually being updated as new counts are completed.

• Gavel
  
• Previcur Flex
  
• Ranman
  
• Tanos
  

Alternate these products and mix each with either mancozeb or chlorothalonil (i.e. Bravo). This development will be watched carefully and new information and recommendations posted.

As downy mildew moves across the state (see map), cucumber growers can be certain that their crop will become infected if it isn’t protected. With a few more weeks until the end of the growing season, it is unlikely that unprotected cucumbers will escape this disease.

Downy mildew has also been reported on muskmelon and winter squash in Michigan . Downy mildew on pumpkin has been reported in Ohio and New Jersey . Fungicides will also be needed for the duration of the growing season to protect these crops from downy mildew. This disease is extremely destructive and can kill an unprotected crop within 7 to 10 days.

• Spray 1: Previcur Flex (1.2 pt.) + Bravo (or Mancozeb)
• Spray 2: Tanos 50 DF (8 oz.) + Mancozeb (or Bravo)

All products listed above must be used at full label rates. As a mixing partner, Mancozeb is good against downy mildew but carries a five-day pre-harvest interval. This makes Mancozeb use especially difficult for growers of hand-harvested cucumber, zucchini and summer squash. Remember, Mancozeb is not registered for use on pumpkins, but Maneb can be used. Some growers are using copper as a mixing partner with Previcur Flex or Tanos 50 DF when the pre-harvest interval prevents the use of Mancozeb.

Ranman (2.75 fl oz) + Mancozeb can also be included in the program of Previcur Flex + Bravo (or Mancozeb) alternated with Tanos 50 DF + Mancozeb (or Bravo) if the downy mildew is not yet present in the field. Last year, when Ranman was included in a field trial where the disease was well established, it did not look good. However, growers of hand-picked cucumbers are in a real bind because of pre-harvest intervals. Previcur Flex has a two-day pre-harvest interval, Tanos 50 DF has a three-day pre-harvest interval, and Mancozeb has a five-day pre-harvest interval. Ranman has a zero-day pre-harvest interval and should be a big help to those needing to get into the field to harvest. However, Ranman must be used prior to downy mildew infection for best results.

Fungicides will be needed for the duration of the growing season to protect cucurbit crops from downy mildew. This disease is extremely destructive and can kill an unprotected crop within 7 to 10 days. Since downy mildew is spreading across Michigan, no one should assume that their crop will not become infected.

There are three elements mandatory for a successful downy mildew program:

1.Use only those fungicides proven to be effective. Data from Michigan State University shows the following program works.

• Spray 1: Previcur Flex (1.2 pt.) + Bravo (or Mancozeb)
• Spray 2: Tanos 50 DF (8 oz.) + Mancozeb (or Bravo)

All products listed above must be used at full label rates. Cutting rates will leave the crop vulnerable to downy mildew. As a mixing partner, Mancozeb is good against downy mildew but carries a 5-day pre-harvest interval. This makes Mancozeb use especially difficult for growers of hand-harvested cucumber, zucchini and summer squash. Remember, Mancozeb is not registered for use on pumpkins, but Maneb can be used. Some growers are using copper as a mixing partner with Previcur Flex or Tanos 50 DF when the pre-harvest interval prevents the use of Mancozeb. Copper should not be applied under extremely hot conditions or when the weather prevents rapid drying on the leaf surface.

I’ve received a number of questions regarding additional fungicides and their use against downy mildew. I have a large 25-treatment downy mildew trial currently underway. My preliminary observations indicate that Ranman (2.75 fl oz), a relatively new fungicide, can be included in the program of Previcur Flex + Bravo (or Mancozeb) alternated with Tanos 50 DF + Mancozeb (or Bravo) if the downy mildew is not yet present in the field. Last year, when Ranman was included in a field trial where the disease was well established, it did not look good. However, growers of hand-picked cucumbers are in a real bind because of pre-harvest intervals. Previcur Flex has a 2-day pre-harvest interval, Tanos 50 DF has a 3-day pre-harvest interval, and Mancozeb has a 5-day pre-harvest interval. Ranman has a 0-day pre-harvest interval and should be a big help to those needing to get into the field to harvest. However, Ranman must be used prior to downy mildew infection for best results.

2. Keep the spray interval short. Cucurbits grow rapidly and the new growth must be protected. That means a 5-day spray interval for many growers is the only option. Especially growers near the hardest hit areas of the state (see map) must assume that the downy mildew spore load is high (see spore chart) and so is the threat of disease. Those growers with the most severe downy mildew problems have stretched their spray interval and left their crop unprotected at critical times. Remember, that if you use a fungicide in your rotation that is not effective against downy, you also leave your crop vulnerable because the interval between the fungicides that are working becomes too long. The extremely high temperatures of the last several days will limit the spore production of the downy mildew, but the pathogen will rev back up again as soon as the temperatures go back to a seasonal level.

3.Use ground sprayers. Some growers are reporting downy mildew problems in fields that have been sprayed with airplanes. Fungicides for downy mildew control should be applied with ground rigs in a volume of water of at least 30 gals. Using a higher amount of water such as 50 gallons is preferred to ensure uniform and thorough plant coverage.

Note: Check my website for updates to this table. (http://www.plantpathology.msu.edu/labs/hausbeck/hausbeck.htm)

All cucumbers in the state are now at a VERY HIGH RISK for downy mildew. Spray programs for cucumbers must be tight and not exceed a five-day application interval. The recommended fungicide program remains unchanged:

Apply: Previcur Flex (1.2 pint) + Bravo (or Mancozeb or Maneb)
Alternate with: Tanos 50DF (8 oz.) + Mancozeb (or Maneb or Bravo)
Remember the pre-harvest intervals (PHI) for these products:

• Tanos – 3 days PHI
• Previcur Flex – 2 days PHI
• Bravo – 0 days PHI
• Mancozeb, Maneb – 5 days PHI

All other cucurbits including cantaloupe, hard squash, zucchini, and pumpkins are now at HIGH RISK for downy mildew. I recommend that the fungicide program listed above be applied at five to seven-day intervals. (Please note that mancozeb is not registered on pumpkins but maneb may be used.). In Michigan, downy mildew has been confirmed on cantaloupe in Lenawee County. Hard squash in the Tuscola County area also have downy mildew. Ohio State has reported downy mildew on pumpkin, hard squash, cantaloupe and summer squash. New Jersey has reported downy mildew on pumpkin.

Downy mildew is becoming widespread on cucumbers in Michigan with several counties experiencing significant outbreaks. Just this week, six new Michigan counties have been confirmed including the eastern and western regions of the state (see map). In these regions, several large fields are infected. The infections appear to be fairly new. Although the infections apparently occurred recently, the amount of spores being produced on the undersides of the leaves is especially high.

The weather is not helping us keep the downy mildew contained. The overcast, wet and humid weather favors this disease and promotes massive production of spores that can be moved to new growing areas. Compared to last year, the weather this year is much more favorable for downy mildew. Not only is downy mildew here earlier this year than last year, but the weather is much more favorable this year than last. I think a perfect downy mildew storm is brewing.

Are the spore traps working?
The spore traps are best suited to monitor the spore load in a field that is already infected. Given the nature of the spore trapping, there is always a 7-day lag period until the spore reel is retrieved from the field and then additional time is needed to process the tapes and count the spores. Currently, the spore trap in Monroe is in an infected cucumber field. In the other counties, the spore traps are in fields that currently do not have downy mildew. I’m still in the research mode with these spore traps and it’s quite possible that they cannot be used for early detection (see accompanying table).

Please remain vigilant and continue to look for any downy mildew symptoms on all cucurbit crops (see pictures). Each day can bring a report of a new outbreak. All samples must be confirmed by my lab. This will keep false reports to a minimum. My lab (517-355-4576) and Diagnostic Services (517-355-4536) on campus are available to look at any potential downy mildew samples. I can be reached via cell phone at 517-927-4532.

NOTE: Check my web site for updates to this table. (http://www.plantpathology.msu.edu/labs/hausbeck/hausbeck.htm)

Reports of downy mildew continue to increase both within southeast Michigan and nearby states. In Michigan, the outbreak thus far is still limited to southeast Michigan. Currently, there are two farms in Monroe with a third farm near the Lenawee/Monroe County border confirmed late last week. Only cucumber fields have been confirmed with the disease thus far. The recommended fungicide programs are working. However, in the latest downy mildew case, the recommended fungicides for downy mildew had not been used and the field is nearly 100% infected. Remember, early applications of fungicide are critical when the spore load is high and weather is favorable (overcast and humid/wet). For instance, last week there were no symptoms on emerging cucumber seedlings in southeast Michigan on July 12 th. On July 18 th, the first true leaves were showing on 60% of the crop with the remainder of the crop still emerging. Downy mildew symptoms are now obvious on these first true leaves. (See photos).

A downy mildew outbreak has been confirmed in northern Kusciusko County, Indiana. The location of this particular field increases the risk to growers in western Michigan. Also, keep in mind that it is very likely that home gardens and other small production fields for roadside markets have downy mildew. In fact, I think you can bet that downy mildew is ravaging pockets of cucumbers growing in backyards. These fields are not being reported, and it is likely that the homeowner or grower does not realize what is happening. Such situations are being reported in a nearby region. As the season progresses, fields with uncontrolled downy mildew will produce large spore loads allowing downy mildew to spread. Spores are produced in highest numbers when the temperature is between 59° and 68°F. However, spores can be produced in low numbers at higher temperatures (up to 86°F).

High spore loads are occurring in southeast Michigan (see Table 1). Spore loads in other areas of the state are negligible. With the number of fields affected in Ohio and Ontario, it seems reasonable to assume that the spore loads in these regions are also high. As downy mildew builds in the region and creeps west, controls will need to be tightened heading into the August growing season. If you cut fungicide rates, use products other than those recommended (see above), or stretch spray intervals during wet weather, your risk of downy mildew will increase (a lot!). Trying to play catch up with downy mildew is a losing proposition! The risk of downy mildew in the state is probably too high now to stretch the fungicide interval beyond 7 days or to use the mancozeb fungicide alone. Cucumber growers in southeast and south central Michigan will need to be on a 5-day application interval to ensure the needed protection against this aggressive and fast-moving disease. Cucumber growers in the other regions of the state should consider a 5-day application interval during periods of overcast and wet/humid conditions. A 5-day interval will help protect a crop that is not emerging uniformly. While I am concerned about the cost of a 5-day application program, the spore load that is available in this state alone is worrisome.

At this point, it seems that cucumbers are especially at risk for downy mildew. There have not been any reports of this disease on melons and that is somewhat surprising. Based on what we know about downy mildew, the pathotypes that affect cucumbers also go to cantaloupes. Are we dealing with a new pathotype? That is a question that will not be answered this growing season. Therefore, protecting cantaloupes against downy mildew as previously recommended is a prudent course of action. It is less likely that watermelon, squash, or zucchini will become infected with downy mildew. However, growers and scouts must remain vigilant and watch all cucurbit crops carefully for any potential downy mildew symptoms.

It is astounding how many new reports of downy mildew have been made from important cucumber growing regions in the last three days. Several fields in Ohio are now showing symptoms although the fungicide program is keeping the disease in check. Ontario is not faring as well. They are experiencing an outbreak that is not being controlled. It seems they do not have the same effective fungicide tools that United States growers have. Further, Delaware has now reported the first occurrence of downy mildew in that state. To my knowledge, cucumber is the only crop that has been reported with downy mildew.

In Michigan, downy mildew was confirmed on July 7 in a 35-acre cucumber field in Monroe County. This report added to the reports last week of downy mildew on cucumbers in northeast Ohio and Ontario. Surely, the period of extended wet, humid weather that occurred approximately two weeks ago has played a key role in these new outbreaks.

Currently, the newly identified 35-acre cucumber field in Michigan has a very low level of downy mildew because of a good fungicide program (only two infected leaves could be found). This farm is relatively close (2 to 3 miles) to the Michigan farm with the earlier downy mildew outbreak.

What to look for
Downy mildew causes symptoms on the leaves similar to a mosaic or angular leaf spot. The tell-tale symptom of downy mildew is the dark, purplish/gray fuzz on the underside of the leaf, giving a somewhat “dirty” or “velvety” appearance (see picture). This fuzz may be most evident in the morning. Leaves of all ages can become infected, including cotyledons.

What to do if you think you have it
Contact your local MSU Extension office immediately. All samples must be examined by someone in my laboratory or by Dr. Jan Byrne in the MSU Diagnostic Services for downy mildew to be confirmed.

Based on MSU trials from last year, Previcur Flex and Tanos 50 DF are the recommended anchor products in a downy mildew fungicide program. These products should be used in alternation so that the downy mildew does not develop resistance. To further delay the development of resistance and enhance the ability of the anchor products (Previcur Flex, Tanos 50 DF) to control disease, a protectant fungicide (mancozeb or Bravo, for example) should be added to each spray.

• Spray 1: Previcur Flex (1.2 pt.) + Bravo (or Mancozeb)
• Spray 2: Tanos 50 DF (8 oz.) + Mancozeb (or Bravo)

If Previcur Flex is not readily available, two Tanos 50 DF applications can be made in a row prior to changing up the spray program with Previcur Flex. Overall, fungicides are more likely to be effective when applied prior to the appearance of downy mildew. Tanos 50 DF has a 3-day PHI and Previcur Flex has a 2-day PHI. The addition of Mancozeb increases the PHI to 5 days. Bravo has a 0 day PHI.

If you grow watermelons, squash, or pumpkins
Frequent applications (every 7 days) of Mancozeb may be adequate but as the pressure builds in the state, I’m inclined to suggest using Tanos 50DF in alternation with Previcur Flex in the program as well (Note: Remember that Mancozeb is not labeled for use on pumpkin, but you can use Maneb/Manex). If this seems wishy-washy, the problem is that we don’t know whether the downy mildew pathotype in the state can infect these crops. Since all of the pathotypes that infect cucumber also infect cantaloupe, the spray recommendations are obvious for cantaloupes. The waters become murky after that. For instance, two out of the five downy mildew pathotypes that infect cucumbers (and cantaloupe) also infect watermelon. Only one of the five downy mildew pathotypes that infect cucumber (and cantaloupe) can also infect squash and pumpkin.

Here are some thoughts as you consider whether you can justify the cost of a more robust downy mildew program for watermelons, squash, and pumpkins:
There is more time this year than last year for the downy mildew to build up in the state.

It is still very early in the growing season. This is different than last year when downy mildew was detected in August. Downy mildew infections early in the season translate into large yield and quality losses.

Adding Tanos and Previcur Flex to the spray program offers protection from downy mildew and other pathogens that can affect these crops each year in Michigan. Not the least of these is Phytophthora capsici! Since Phytophthora is a cousin to downy mildew, the fungicides that are recommended for downy mildew also offer some control of Phytophthora crown and fruit rot.

You cannot play catch up with downy mildew. Fungicides must be applied preventively to work well.

Previcur Flex or Tanos 50 DF were outstanding products in our MSU trial last year (Note: the Curzate fungicide has one of the active ingredients that is present in Tanos 50 DF and can be used in place of Tanos 50DF). If Curzate is used, it should be mixed with either Bravo or Mancozeb. Curzate should not be used in alternation with Tanos 50DF because they have an important active ingredient in common.

• Weather – Is the weather favorable for downy mildew with rain, high humidity, fog, long dew periods?
• Air currents – Will the air current continue to flow to the east?
• Disease spread – Will downy mildew move beyond southeast Michigan into other areas of the state? Will it infect watermelon, squash, or pumpkin?
• Stage of the crop – Is it close to harvest? The closer you are to the finish line, the better.

While downy mildew is not widespread in the state at this point, each day can bring a report of a new outbreak. We must remain vigilant! Walk those fields, send in suspicious samples, let’s work together to get through this season. My lab (517-355-4576) and Diagnostic Services (517-355-4536) on campus are available to look at any potential downy mildew samples. I can be reached via cell phone at 517-927-4532.

The threat of downy mildew continues to drive the spray recommendations. Last Friday, the North Carolina State University Cucurbit Downy Mildew Forecast issued the following prediction for southeast Michigan:

Serious Threat of downy mildew developing for Thursday (June 22), Moderate Threat for Friday (June 23), and Low Threat for the weekend (June 24, 25).

If this week’s weather forecast holds (cloudy and rainy), it is likely the downy mildew threat will increase again.

What about the threat of downy mildew to Michigan growers outside of the southeast region? According to the Cucurbit Downy Mildew Forecasts that have been generated, the southeastern portion of Michigan has the highest risk. However, that does not mean that the other regions of the state have a zero chance of disease.

Downy mildew also threatens other growing regions outside of Michigan. Last week, the downy mildew risk was high for northern Ohio and southern Ontario and moderate for southwest and south-central New York and northern Pennsylvania.

These forecasts are based on the downy mildew biology, weather predictions and the trajectory of the air currents carrying the spore loads. The mat of velvet that forms underneath the leaves are actually masses of spores that are commonly referred to as sporangia. These spores are formed during the night when the leaves become wet with dew, rain or fog. The spores are produced on spore stalks. The spores mature (or ripen) during the early morning hours of (1:00 to 5:00 AM). As the morning progresses, the leaves begin to dry, the relative humidity falls and a breeze may begin to blow. As the morning dew dries, the spore stalk begins to dry and twist, dislodging the mature (ripened) spore from its stalk and placing it into air currents (See graph). The air currents can then move these spores long distances. Rain showers are thought to scrub these spores from the atmosphere and deposit them onto fields. Once the spores are dropped from the air onto the foliage, wetness is needed for the spore to germinate and cause a new infection.

Fungicides are most effective when they are applied prior to the spores landing on the leaf’s surface. Once the spores have germinated and penetrated the leaf, it becomes much more difficult to limit downy mildew. A preventive approach will always be more effective than trying to stop the disease once it has appeared.

Spore traps have been placed in six Michigan counties and include: St. Joseph, Allegan, Monroe, Bay, Van Buren and Saginaw counties. The spore load has been relatively high from Monroe County during certain days when the weather was favorable for spore production. The downy mildew spores detected in the other spore trapping regions in the state have been negligible. Please remember that the spore traps may not be in the correct site to pick up all influxes of downy mildew spores into a region. At this point, we are using these traps as a research tool. Any information that we learn from these traps will be shared over the course of the growing season.

Growers of pumpkins, squash, or watermelons in the southeast Michigan region should be on a 7-10 day alternation program.

Alternate these sprays:

• Previcur Flex (1.2 pint) + Bravo (or Mancozeb)
• Tanos 50DF (8 oz.) + Mancozeb (or Bravo)

Please note: Mancozeb is not registered for use on pumpkins, but Maneb or Manex could be used.

If Previcur Flex is not readily available, two Tanos 50DF applications may be made in a row prior to changing up the spray program with Previcur Flex. The fungicides will be more effective when applied prior to the appearance of downy mildew. Tanos 50DF has a 3-day pre-harvest interval (PHI) and Previcur Flex has a 2-day PHI. The addition of Mancozeb increases the PHI to 5 days. Bravo has a 0-day PHI.

At this point, downy mildew appears to be isolated to Monroe County. No new fields have been reported to have the disease. It is possible that the two fields that are currently diseased will be out of production sometime next week and will be burned down. Growers of cucumber and/or cantaloupes in the rest of the state have a lower risk of downy mildew developing than the growers in southeast Michigan who are in the immediate vicinity of the infected fields. How low is the risk to the Michigan areas outside of Monroe County and what should be done to protect cucumbers and melons in the northern and western parts of the state?

The fungicide program that offers the least risk of downy mildew developing also carries the highest cost: a 7-10 day alternation program with the fungicides Previcur Flex (1.2 pint) + Bravo (or Mancozeb) alternated with Tanos 50DF (8 oz.) + Mancozeb (or Bravo).

A fungicide program that offers a moderate level of risk with a more moderate price tag includes a mancozeb only program applied every 7 to 10 days. Mancozeb is available under the trade names of Dithane, Penncozeb, Manzate and others. Please note: Mancozeb is not registered for use on pumpkins, but Maneb or Manex could be used. The thought behind using this program is that if a downy mildew outbreak develops in the growing region, Previcur Flex and/or Tanos would be added to the mancozeb immediately.

While some growers have not applied any fungicides, the relatively high use of fungicides early on likely prevented the spread of this disease to nearby young cucumber and melon fields. Although the disease was initially detected on June 9, the downy mildew was well established and was likely present in the two fields as early as mid-May. We will continue to handle any suspect samples of downy mildew and will visit any fields with unusual symptoms.

Cucumber growers anticipate another year of downy mildew infestation in their crops. There have been questions about mixing postemergence herbicides and fungicides for control of broadleaf weeds and diseases in a single pass across the field.

The primary postemergence broadleaf herbicide for cucumbers is Sandea (halosulfuron). It is formulated as a 75DF. Mixing Sandea with fungicides with flowable or dry flowable formulations should not increase crop injury. Emulsifiable concentrate (EC) formulations usually increase the potential for crop injury in tank mixes, so avoid mixing Sandea with any pesticides formulated as EC.

Before mixing any pesticides, check labels for any cautions or restrictions on tank mixes.

It has been a relatively quiet week with no new cases of downy mildew found. The two cucumber fields with downy mildew have not been destroyed and will likely be in production for the next four weeks. We are working to determine the threat that these fields pose to the cucumber and cantaloupe producers in the state. The diseased fields have been under an intensive fungicide program to keep the downy mildew limited.

The downy mildew pathogen moves from field to field primarily via air currents. It reproduces via tiny, microscopic spores that act as seeds of the pathogen. Spore traps have been placed in one of the diseased cucumber fields and also in four other Michigan counties: St. Joseph, Allegan, Bay, Van Buren and Monroe.

These traps are research tools that are manufactured in England and cost approximately $5,000 to $6,000 each. Inside of the trap there is a drum that has a cellophane tape with a sticky surface. This tape rotates past an intake orifice at the rate of approximately 2 mm per hour. Anything in the air that flows past the cellophane tape becomes embedded in the sticky material on the tape and includes spores, pollen, insect parts, debris, etc. At the end of 7 days, the cellophane tape is removed from the drum, cut into 24mm segments and is mounted onto a microscope slide. A compound microscope is needed to have enough magnification to identify any downy mildew spores that may be present on the tapes. The spore traps will help to alert us to any influx of spores into those production regions. These traps cannot be used as a substitute for the fungicide sprays. Since we do not have a trap in each field, it is possible that we could miss an isolated spore mass coming into a particular region.

Are fungicide sprays still needed? Yes. Many cucumber and cantaloupe growers have now applied two fungicide sprays and have protection through this week. At this juncture, I recommend that growers continue with the same fungicide program. The recent series of rains will be favorable for downy mildew development.

Alternate these sprays:

• Previcur Flex (1.2 pint) + Bravo (or Mancozeb)
• Tanos 50DF (8 oz.) + Mancozeb (or Bravo)

Growers of cucumbers or cantaloupes in southeast Michigan region should be on a 5-day alternation program. Growers in the rest of the state should be on a 7-day alternation program. If Previcur Flex is not readily available, two Tanos 50DF applications may be made in a row prior to changing up the spray program with Previcur Flex. The fungicides will be more effective when applied prior to the appearance of downy mildew. Tanos 50DF has a 3-day PHI and Previcur Flex has a 2-day PHI. The addition of Mancozeb increases the PHI to 5 days. Bravo has a 0-day PHI.

Some later planted fields may be covered with a banded application of fungicide. As further sprays are needed, cost becomes difficult to bear. However, with downy mildew present in the state and the weather providing the moisture needed for the disease, I do not think that we have a choice. Fungicides must be applied preventively and frequently.

Downy mildew causes symptoms on leaves that include angular yellow lesions and brown, dead leaf areas (Figure 1). The tell-tale symptom of downy mildew is the dark, purplish/gray fuzz on the underside of the leaf, giving a somewhat “dirty” or “velvety” appearance (Figure 1). This fuzz, which is the accumulation of spores, may be most evident in the morning. Leaves of all ages can become infected, including cotyledons.

As fields are scouted for downy mildew, it is important to know that there are other diseases and leaf injuries that can look similar to downy mildew.

Look-alike diseases: Angular leaf spot can produce symptoms of angular yellow lesions that are often shiny on the leaf surface due to dried bacterial ooze (Figure 2A). Anthracnose, caused by a fungus, produces tan spots on leaves which die and fall out, resulting in a shot-hole appearance. Neither of these diseases produce spores or a “dirty” appearance on leaf undersides.

Look-alike leaf injuries: Herbicide damage can give leaves the appearance of both yellow mottling (Figure 2B) and brown lesions with dead areas which fall out of the leaf, appearing tattered (Figure 2C). Heat stress can also affect leaves, resulting in pale to white spots visible on leaf surfaces (Figure 2D). Heat stress is particularly a problem on plants that were grown under plastic tunnels and may have experienced extreme temperatures. Many Michigan cucumber fields have had cucumber beetle activity during the last 2 weeks. Beetle feeding on leaf undersides can result in the appearance of yellow to brown leaf areas, and when feeding damage is fresh, damaged leaf undersides can look dark green and wet. None of these leaf injuries result in the appearance of “dirty” leaf undersides.

Downy mildew was confirmed on June 9 on slicing cucumbers in Monroe County. Currently, there are two 8- to 10-acre fields with advanced disease. These two fields were planted in late April by direct seeding and plugs grown in an on-site greenhouse. At this time, there is no visible disease on nearby cantaloupe plantings. Other cucumbers in the Southeast area are only recently emerged and are not diseased. At present, cucumbers and cantaloupe are most at risk. It is less likely that pumpkins and squash will become infected by downy mildew. At this time, it is not known whether this is an isolated incident or more widespread problem. Many cucumber fields in other regions of the state have been scouted and no disease has been found.

What to look for
Downy mildew causes symptoms on the leaves similar to a mosaic or angular leaf spot. The tell-tale symptom of downy mildew is the dark, purplish/gray fuzz on the underside of the leaf, giving a somewhat “dirty” or “velvety” appearance. This fuzz may be most evident in the morning. Leaves of all ages can become infected, including cotyledons.

• Spray 1: Previcur Flex (1.2 pt.) + Bravo (or Mancozeb)
• Spray 2: Tanos 50 DF (8 oz.) + Mancozeb (or Bravo)

Growers of cucumbers and/or cantaloupes outside of the southeast Michigan region should also use the above program but can use a 7-day spray interval.

Many growers have reported that Previcur Flex may not be readily available. In that case, lead off with Tanos 50 DF. Two Tanos 50 DF applications can be made in a row prior to changing up the spray program with Previcur Flex. Overall, fungicides are more likely to be effective when applied prior to the appearance of downy mildew. Tanos 50 DF has a 3-day PHI and Previcur Flex has a 2-day PHI. The addition of Mancozeb increases the PHI to five days. Bravo has a zero-day PHI.

If you grow watermelons
An application of mancozeb is adequate at this time while the disease situation in the state is assessed.

If you grow squash and pumpkins
Fungicide sprays are not critical at this time. Stay tuned for a change in this recommendation.

Previcur Flex or Tanos 50 DF were outstanding products in our MSU trial last year. (Note: the Curzate fungicide has one of the active ingredients that is present in Tanos 50 DF and can be used in place of Tanos 50DF). If Curzate is used, it should be mixed with either Bravo or Mancozeb. Curzate should not be used in alternation with Tanos 50DF because they have an important active ingredient in common.

See http://www.ipm.msu.edu/cat06veg/pdf/downymildew.pdf for results of last year's downy mildew fungicide trial.