Time for downy mildew protectant sprays for cucumbers

As we near the beginning of July, it’s important to note this is typically the time of year we expect to find the first downy mildew symptoms on cucumber. Downy mildew is a destructive disease that causes rapid death of the foliage if it isn’t controlled. In Michigan, I have seen that cucumber and melon are most vulnerable to the disease. In some years, hard squash foliage has also become infected.

The symptoms on cucumber resemble those caused by angular leafspot because the lesions are not round but are squared off by the leaf veins (Fig. 1). The downy mildew symptoms on melon, pumpkin and squash can look different than those on cucumber (Fig. 2B-E). During dewy mornings or rainy days, early leaf infections may look dark green and water-soaked. These dark-green areas turn yellow in a few days’ time and then become brown. The underside of the lesion takes on a dark, fuzzy appearance due to the downy mildew spores that develop. It is these dark, downy mildew spores that are released into the air where they may be carried to new cucumber plantings resulting in more outbreaks.

(A) Spore trap for monitoring airborne downy mildew spores. Downy mildew on: (B) cantaloupe, (C) pumpkin, (D) watermelon and (E) squash.

Although the downy mildew pathogen does not overwinter in previously affected fields in the state due to the cold winter temperatures, it can survive in greenhouses. Each year, the pathogen must arrive to cucumber fields via air currents. For several years, Michigan State University has offered the cucumber growers an early warning system via spore traps (Fig. 2A) that are placed in several counties. These spore traps sample the air and the downy mildew spores are trapped onto a sticky tape. The number of downy mildew spores are totaled for each day and reported.

While the overall spore counts have been low, there was a day with a spore number that was higher than expected for this time of year. Further, the weather during the last 72 hours has been especially favorable for downy mildew development with cool and wet conditions. From past experience with the spore traps, the fact that we are picking up spores on a fairly regular basis indicates it is important to consider using a preventive downy mildew fungicide immediately. Since we haven’t confirmed downy mildew in the state at this time, a broad-spectrum protectant such as chlorothalonil or mancozeb could be considered. A downy mildew-specific fungicide as listed in the table below could be used after the disease is confirmed in the state.

Each year, extensive MSU fungicide trials are conducted to help guide the spray recommendations and determine whether the fungicides are still effective. The most current recommendations are included in the table below (also see Fig. 3). Follow label recommendations for resistance management.

Figure 3. Downy mildew on (A) untreated cucumber plants, and plants treated with (B) Orondis Opti.

Preferred downy mildew fungicides for cucurbits
Product	Active ingredient	FRAC	Comment (maximum applications/season)
*Orondis Opti	oxathiapiprolin/chrothalonil	49/M05	Use either soil applications of Orondis or foliar applications of Orondis Opti but not both for disease control. Do no use more than 1/3 of the total foliar fungicide applications. (6)
*Elumin SC	ethaboxam	22	Mix with chlorothalonil or mancozeb. (2)
Ranman 4SC	cyazofamid	21	Mix with chlorothalonil or mancozeb. (6)
Gavel 75DF	mancozeb/zoxamide	M03/22	(8)
Zampro 4.4SC	ametoctradin/dimethomorph	45/40	Labeled for application via drip or as a foliar spray. Mix with chlorothalonil or mancozeb. (3)
Zing! SC	zoxamide/chlorothalonil	22/M05	(8)

 

–Mary Hausbeck, Michigan State University

Photo at top: A) Top side of cucumber leaf with yellow lesions and necrosis defined by the veins. (B) Underside of cucumber leaf displaying dark, fuzzy spore masses. Photos: Mary Hausbeck/MSU.