Powdery and downy mildew in cucurbits

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

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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