Field Scouting Guide: Cucurbit Downy Mildew

Field Scouting Guide: Cucurbit Downy Mildew

Historically, Michigan growers produce more than 1.4 million tons of cucurbits valued at about $83 million on 43,000 acres. Michigan ranks No. 1 in the nation for production of pickling cucumbers, and in the top six for fresh market cucumber and fresh market/processing pumpkin and squash.

Cucurbit downy mildew (DM) re-emerged as a problem on Michigan cucumbers in August 2005 when the disease spread across the eastern region of the U.S. and has recurred annually since then.

Cucurbit Downy Mildew

Scientific name: Pseudoperonospora cubensis
Crops affected: cucumber, watermelon, cantaloupe, honeydew, zucchini, gourd, summer and winter squash, and pumpkin.


Identifying a Potential Problem

DM causes symptoms on the leaves similar to angular leaf spot. Yellow lesions may be visible on the top surface of infected leaves are there figures.

Look for yellow lesions on top of leaves, and gray-to-black fuzz under.
Photo courtesy of Michigan State University

The telltale sign of DM is the gray to black fuzz on the underside of the leaf giving a somewhat “dirty” or “velvet” appearance. This fuzz may be most evident in the morning.

DM is well-known for causing catastrophic losses in a brief period of time. Ps. cubensis is an obligate biotroph, meaning it cannot live long without a host plant. This condition restricts the pathogen to warmer climates during the winter months, including southern states and greenhouses.

DM spreads to surrounding fields on air currents via tiny, microscopic spores that act as seeds of the pathogen. Cool (~ 60°F), wet, and cloudy conditions create an ideal environment for DM spores to survive outside the host. When the conditions are favorable, unprotected foliage can become completely blighted within 14 days of the initial infection, resulting in catastrophic yield losses.

To help achieve early detection of airborne spores, volumetric spore traps have been placed in Michigan counties during the growing season. Spore traps continuously sample the air and collect spores by imbedding them on a film that is removed and taken to the laboratory for identification and quantification. A compound microscope is used to identify/count Ps. cubensis spores that are present on the film. The spore traps help us to detect an influx of spores into those production regions where the spore traps are located. Thus, when spore concentrations are high, alerts can be issued for growers to begin their fungicide spray program.

Mind Your Management Methods


Several fungicides have tested well for efficacy.
Shown here (clockwise, from upper left corner): Untreated sample; Treated with Oronids; Treated with Zing!; Treated with Ranman.
Photos courtesy of Michigan State University

DM must be managed through a fungicide spray program. Before the DM outbreak of 2005, the disease was effectively controlled through host resistance. Since 2005, the formerly-resistant cultivars have showed slower progression of the disease; however, no current cucumber cultivar has been identified that exhibits complete DM resistance.

A fungicide management strategy should include application of the most effective products. The Hausbeck Lab continues to evaluate new and existing products annually to determine the most effective fungicide products available for DM control.

Research has found that the DM pathogen may be resistant to fungicides that were once extremely effective. Rotating among FRAC groups (different modes of action) is imperative to delay development of resistance in the DM pathogen to new chemistries.

2018 Fungicide Recommendations

The table below lists the products that have tested effective against DM in replicated field trials. Each product should be mixed with a protectant (chlorothalonil or mancozeb) or other DM fungicide.


Active ingredient FRAC


*Orondis Opti SC (Syngenta) oxathiapiprolin/ chlorothalonil 45/M05 Make no more than two sequential applications before rotating to a fungicide with a different mode of action (FRAC). Use either soil applications of Orondis or foliar applications of Orondis Opti A, but not both for disease control. Do not use for more than 1/3 of the total foliar fungicide applications.
*Elumin SC (Valent USA) ethaboxam 22 Alternate with a non-FRAC Group 22 fungicide. Mix with chlorothalonil or mancozeb.
*Ranman 4SC (Summit Agro) cyazofamid 21 Alternate with a non-FRAC Group 21 fungicide. Mix with chlorothalonil or mancozeb.
Zampro 4.4SC (BASF) ametoctradin/ dimethomorph 45/40 Labeled for application via drip or as a foliar spray. Make no more than two sequential applications before rotating to a fungicide with a different mode of action (FRAC). The addition of a spreading/penetrating adjuvant is recommended. Mix with chlorothalonil or mancozeb.
Gavel 75DF (Gowan) zoxamide/mancozeb 22/M03 Mix with chlorothalonil or other DM fungicide.
Zing! SC (Gowan) zoxamide 22/M05 Make no more than two  sequential applications before rotating to a fungicide with a different mode of action (FRAC). Mix with chlorothalonil or other DM fungicide.

*These products have performed exceptionally well in Michigan trials.

Editor’s note: Drs. Hausbeck and Linderman updated this article, which was originally published on Michigan State University’s site.