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| Powdery and downy mildew in cucurbits Powdery mildew on pumpkin Powdery mildew is perhaps one of the easiest diseases to identify because of the whitish, talcum-like, powdery growth that develops on leaf surfaces, petioles, and stems. Infected leaves usually wither and die. Premature loss of foliage often reduces the size or number of fruit and the length of the harvest period. In addition, powdery mildew infection predisposes plants to other diseases such as gummy stem blight. Powdery mildew occurs each year, although the time of disease appearance is unpredictable. Propagules responsible for infection (conidia or spores) may be transported rapidly over long distances by air currents. Therefore, the disease may become established in a clean field from conidia blowing in from a field affected by the fungus. It is also possible that this disease may overwinter in soil and plant debris in a hearty, weather-resistant fungal structure (cleistothecium). Although this has not been verified for Michigan, overwintering of the fungus responsible for powdery mildew has been documented in nearby states. Once powdery mildew is present, the disease can increase rapidly. The fungus can multiply and spread quickly under favorable conditions because the length of time between infection and symptom appearance is usually only three to seven days. Also, a large number of conidia that can infect healthy tissue can be produced in a short time, and contribute to spread of the disease within a field. Currently, fungicides are the primary control practice for this disease. Resistant cultivars are becoming more widely available. Many products were tested for their ability to control powdery mildew (Figures 1 and 2). To avoid the development of resistance, fungicides should be used in alternation. Since Flint, Quadris, and Cabrio affect the powdery mildew similarly, they should not be used in alternation with each other. Rather, they could be used in a program with other fungicides. In some fields, powdery mildew has developed resistance to Nova and the fungicide is ineffective. We believe this was the case at our Michigan State University research farm (Fig. 1). However, in other fields, such as our southwest Michigan site in 2005, the powdery mildew fungus appeared to be relatively sensitive to the Nova fungicide and disease was limited compared to the untreated control (Figure 2). It is critical that a field be monitored closely for the first appearance of the disease. To monitor effectively, a grower must walk through a field once or twice a week to look for powdery mildew, especially on the older, shaded leaves. Do not forget to look at the underside of the leaves. It is apparent from field observations that early control of powdery mildew is the most effective. Downy mildew on cucumber Downy mildew was confirmed in Michigan on August 5, 2005 and is a new disease for the state. The disease developed in several major vegetable production regions, with the majority of reports on pickling cucumber. Downy mildew is very different from powdery mildew. The tell-tale symptom of downy mildew is the dark, purplish/gray fuzz on the underside of the leaf giving a somewhat ‘dirty’ or ‘velvet’ appearance (Figure 3B). The fuzzy signs of the pathogen are most evident on leaf undersides in the morning. Powdery mildew is white and less fluffy. On the leaf surface, downy mildew symptoms appear similar to those of a mosaic or angular leaf spot (Figure 3A). Downy mildew is well-known for causing catastrophic losses of cucurbits in a brief period of time. When it is overcast, cloudy, and humid, unprotected foliage can become completely infected and appear to be frosted within 10 days of infection. The pathogen appears to have come to Michigan in upper air currents from other vegetable production regions of the United States. Downy mildew is not known to produce overwintering spore structures and will not persist in soil and field debris in Michigan from year to year. Currently, there are few cultivars with adequate resistance to downy mildew. Chemical control must be focused on using the most effective products, alternating the products, and applying the fungicides at short intervals. Results from our 17-product downy mildew spray trial indicated that the most effective spray programs include the following: Previcur Flex (propamocarb hydrochloride, Bayer CropScience) plus Bravo (chlorothalonil, Syngenta Crop Protection) alternated with Tanos 50DF (cymoxanil + famoxadone, DuPont Crop Protection) plus mancozeb (or Tanos + Bravo). Our study was conducted in a young pickling cucumber field that was already showing early downy mildew symptoms and sporulation. Overall, fungicides are more likely to be effective when applied prior to the appearance of the pathogen. Previcur Flex is a critical component of the fungicide program. It appeared to be especially effective and offered a different mode of action from that of Tanos 50DF. Tanos 50DF has a three-day PHI and Previcur Flex has a two-day PHI. The addition of mancozeb increases the PHI to five days. Bravo has a zero-day PHI. In addition to fungicide application, it was recommended that any infected vines remaining after harvest should be killed with an herbicide or plowed under immediately so that they do not serve as a source of downy mildew for nearby crops. In a 43-acre pickling cucumber field with initial downy mildew infection at the first true-leaf stage, the recommended Previcur Flex spray program turned the field around to yield 170 bushels per acre. |
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| Downy mildew update Mary Hausbeck Plant Pathology Since last week’s Vegetable CAT Alert, downy mildew has been confirmed on cucumber at three new sites, Saginaw County on July 20, Bay County on July 21, and on a 4 site in Monroe County on July 24 (see map). Fungicide recommendations remain the same.
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Downy mildew confirmed in Lenawee County
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, click on link to Downy mildew news). This table is continually being updated as new counts are completed.Table 2. Daily spore trap totals by county and year. Table 2. Daily spore trap totals by county and year.
DMDisease was found in the county on this date. *Spores present but soil prevented accurate count. ‡Partial day. |
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Downy mildew confirmed in Gratiot County
Table 1. Recommended products for managing downy mildew on cucumber and all cucurbits.
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.
S = Spore trapping started on this date. D = MDisease was found in the county on this date. *Spores present but soil prevented accurate count. |
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| Statewide downy mildew sprays recommended Downy mildew fungicide sprays should be applied to all cucumbers in the state immediately! Fungicides should be applied to cucumbers every five to seven days. Pumpkins, melons, cantaloupes, squash, zucchini and gourds should also be protected with downy mildew fungicides. Fungicides to these crops should be applied every 7 to 10 days. Table 1. Recommended products for managing downy mildew on cucumber and all cucurbits.
Use fungicides proven to work on cucumbers in Michigan! Over the last two years, many fungicide trials have been conducted in Michigan to determine the most effective spray programs. If you use only the recommended downy mildew fungicides at the prescribed spray interval, you will be protected from this potentially devastating disease. Do not use products that are not on this list based on promises from sales staff. Downy mildew confirmed in Michigan Downy mildew on cucumber was confirmed on June 25 in a homeowner’s garden in Monroe County. This garden has since been destroyed. There is also a suspect downy mildew case in Monroe County with a commercial cucumber grower. In this case, downy mildew fungicides had not been applied to this field. I will have a sample from this field late today for confirmation. These cases appear to be the result of relatively new infections that have become established within the last 7-10 days. A weather event may be responsible for the nearly simultaneous downy mildew outbreaks in Michigan, Ohio and upstate New York. In the latest spore trap readings, the downy mildew counts have taken a jump in at least one county. Additional counts are pending and this information will be made available on the downy mildew website associated with my homepage. If you suspect downy mildew please have it confirmed either through my lab (517)355-4576 or through Dr. Jan Byrne at MSU Diagnostics (517)355-3504. To better understand this downy mildew, my laboratory is collecting diseased plants for further analysis. If a sample cannot be sent to the lab, please call and arrangements will be made to pick it up. |
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| Downy mildew update There are no confirmed reports of downy mildew in Michigan. The hot and dry air of the last couple of weeks is not favorable for development of downy mildew. Since there is a report of downy mildew in nearby Ontario greenhouses, cucumber growers in southeast Michigan are advised to begin downy mildew sprays immediately (Table 1). Table 1. Recommended products for managing downy mildew on pickle.
Fungicide trials Chemical control must be focused on using the most effective products, alternating the products and applying fungicides at short intervals. Results from our downy mildew research in 2005, indicate that an effective spray program includes the following: Previcur Flex (propamocarb hydrochloride) plus Bravo (chlorothalonil) alternated with Tanos 50DF (cymoxanil + famoxadone) plus mancozeb. The initial sprays for the 2006 trials were applied when plants had one true leaf and no disease symptoms were apparent. Ten applications were made on August 1, 7, 11, 15, 21, 26 and 31; September 6, 13 and 20 following a five to seven day spray schedule. Plots were visually evaluated for necrotic leaves on September 11 (see Figure 1). Fruits were hand-harvested four times from the entire 15 ft of all treatment rows on 5, 11, 18, and 25 Sep. Products that looked favorable in our 2006 field studies include Ranman 3.6SC (cyazofamid), Gavel 75WG (mancozeb + zoxamide), V-10161 4FL (fluopicolide), Tanos 50WG and Previcur Flex 6SC. Each of these products should be mixed with either Dithane or Bravo (see Table 1). |
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| Downy mildew reported on cucumbers growing in Canadian greenhouses Downy mildew has been confirmed on greenhouse cucumbers in Essex County in Canada. These are not plug plants for field production but are “hothouse” cucumbers. Essex County is Canada’s southernmost county located on a peninsula of land that juts out into the region of the Midwestern United States. At this time, no field outbreaks have been reported in Canada or Michigan. The greenhouse outbreak in Canada is troubling news for Michigan pickle and cucumber growers. The field cucumber growers in Canada are better equipped to handle downy mildew than they were a year ago because of expanded fungicide labels. However, I don’t think that Michigan growers can assume that downy mildew will stay in Canada. The recent dry and bright sunny weather will delay downy mildew development in Canada, for now. My concern is that the downy mildew spores will leave the greenhouse and infect nearby field plantings of cucumbers. If a field of cucumbers becomes infected in Canada, then our risk in Michigan increases. The vegetable crop specialist in Canada is recommending that field cucumber growers in Canada spray preventively. It is my opinion that cucumber growers in Southeastern Michigan should spray fields preventively now. If the growers in the areas of the Midwest closest to Essex County spray diligently, perhaps a 2007 epidemic can be averted. Spore trapping for downy mildew in Michigan began last week with some stations still to be set up this week. Sprays can be banded for fields that are young and newly emerged. Cucumbers that have been growing under tunnels will be larger and will need to be covered completely with spray to be protected. A spray interval of 10 days is currently recommended. The following fungicides are recommended:
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. |
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| Five-day sprays needed for downy mildew on cucumbers Mary Hausbeck Plant Pathology 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
Use only those fungicides proven to be effective Data from
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. Keep the spray interval shortFungicides must be applied to cucumbers every five days for effective results. Cucurbits grow rapidly, and the new growth must be protected. 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. Now that the temperatures have dropped, the downy mildew will reproduce at very high levels. Extended dew periods at night will greatly increase disease. |
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| Downy mildew spray program critical for all cucurbit growers Mary Hausbeck Plant Pathology 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.
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. Table 1. Spore trap daily totals (counts/m 3/day) by location.Note: currently only the spore trap in Monroe County is in a field with downy mildew disease.
**Spore tapes available, counts pending. …Spore trap moved to new location. Note: Check my website for updates to this table. (http://www.plantpathology.msu.edu/labs/hausbeck/hausbeck.htm) |
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| Important update as of July 28 Downy mildew on cukes widespread in Michigan – other crops now infected Mary Hausbeck and Amanda Gevens Plant Pathology 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)
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? 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. Table 1. Spore trap daily totals (counts/m 3/day) by location.
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