Managing Cucurbit Powdery Mildew

Powdery mildew arrived this week on squash, pumpkins and other cucurbits throughout Ohio. It is a little late – we often see it by early- to mid-July.  The fungus that causes cucurbit powdery mildew does not overwinter in Ohio, so the disease does not appear until spores arrive on wind currents from warmer growing areas.  This fungus is an unusual plant pathogen in that it is inhibited by free water – so frequent rains may delay powdery mildew’s appearance, at least to a notable level.  Signs of infection are small circular powdery growths (mycelium and spores of the pathogen) on either side of the leaf. These spots enlarge and can eventually cover most of the leaf surface and kill the leaves.  Stems and leaf petioles are also susceptible, but the disease is not observed on fruit.  In pumpkins, powdery mildew may also attack the “handles”, which can be further damaged by secondary pathogens.

Powdery mildew is managed using powdery mildew-resistant varieties and fungicides.  Development of insensitivity to overused fungicides is common in populations of the fungus that causes this disease, so it is important that a fungicide resistance management program is followed. Remember to alternate fungicides in different FRAC (Fungicide Resistance Action Committee) groups, indicating different modes of action against the fungus. It is important to apply fungicides when the disease first appears and incidence is low. Fungicides that are effective against cucurbit powdery mildew can be found in the Midwest Vegetable Production Guide for Commercial Growers; product ratings and FRAC codes are on page 125.  Our evaluations of efficacy of powdery mildew fungicides at three locations (Wooster, Columbus, South Charleston) in Ohio in 2017 indicated that Procure, Quintec, and Rally consistently provided very good control of powdery mildew on pumpkins in all three locations (see table).  Approvia Top and Inspire Super were very good in two locations but fair in a third; and Merivon Xemium, Fontelis and Torino were very good in one location and fair in two. Both Bravo and Pristine performed poorly in all three locations.

No Cucurbit Downy Mildew Reported Yet in Ohio, but be Prepared

Cucurbit downy mildew reports as of July 12, 2018.

Cucurbit downy mildew is marching up the U.S. eastern seaboard from Florida to central Pennsylvania, but has not been reported in Ohio, Michigan or Ontario as of July 12.  This is later than normal for Ohio – last year our first cucumber downy mildew report was on June 28. Excessive heat followed by sunny days have likely contributed to the delay.  However, cucurbit growers should be vigilant and scout their fields regularly for downy mildew.  These crops should be protected now with an effective protectant fungicide such as chlorothalanil (Bravo, Echo, Equus, Initiate versions), which will also help manage anthracnose and Alternaria leaf spot – we have seen an unusually large amount of Alternaria leaf spots in various crops this summer. When we start experiencing cooler, rainier weather with high humidity and overcast skies, the downy mildew risk will increase.  We evaluated numerous fungicides for efficacy against cucumber downy mildew in 2017 (see chart below).  Ranman 400SC, Orondis Opti, Omega 500F and Gavel 75 DF performed best in these tests.  Bravo Weather Stik 6F, Zampro 52SSC, Tanos 50DF, Presidio 4SC and Curzate 60DF were intermediate in efficacy and can be used as rotational partners in a fungicide program, particularly under low to moderate disease pressure.  The poorly-performing fungicides are not recommended for downy mildew management.  Always rotate fungicides with different modes of action and follow label instructions. Remember that Orondis Opti applications are restricted to 1/3 of the total fungicide applications. Under highly conducive environmental conditions, apply fungicides on a 5-7 day schedule.  When the risk is lower due to hot, dry, sunny weather, or downy mildew has not been reported in the area, the schedule may be stretched to 7-10 days. Cucumber and cantaloupe downy mildew risk is much higher in northern than in central and southern Ohio at this time.

Information on fungicides for vegetables, including Fungicide Resistance Action Committee (FRAC) code and greenhouse use can be found in a table beginning on page 79 of the 2018 Midwest Vegetable Production Guide for Commercial Growers. Pre-harvest intervals are shown for each crop/fungicide combination throughout the guide.