Cucumber downy mildew look-alikes

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.

Downy mildew causes symptoms on leaves that include angular yellow lesions and brown, dead leaf areas (Figure 1). The tell-tale symptom of downy mildew is the dark, purplish/gray fuzz on the underside of the leaf, giving a somewhat “dirty” or “velvety” appearance. This fuzz, which is the accumulation of spores, may be most evident in the morning. Leaves of all ages can become infected, including cotyledons.

Figure 1. Downy mildew symptoms on cucumber

As fields are scouted for downy mildew, it is important to know that there are other diseases and leaf injuries that can look similar to downy mildew.

Look-alike diseases: Angular leaf spot can produce symptoms of angular yellow lesions that are often shiny on the leaf surface due to dried bacterial ooze (Figure 2A). Anthracnose, caused by a fungus, produces tan spots on leaves which die and fall out, resulting in a shot-hole appearance. Neither of these diseases produce spores or a “dirty” appearance on leaf undersides.

Look-alike leaf injuries: Herbicide damage can give leaves the appearance of both yellow mottling (Figure 2B) and brown lesions with dead areas which fall out of the leaf, appearing tattered (Figure 2C). Heat stress can also affect leaves, resulting in pale to white spots visible on leaf surfaces (Figure 2D). Heat stress is particularly a problem on plants that were grown under plastic tunnels and may have experienced extreme temperatures. Many Michigan cucumber fields have had cucumber beetle activity during the last 2 weeks. Beetle feeding on leaf undersides can result in the appearance of yellow to brown leaf areas, and when feeding damage is fresh, damaged leaf undersides can look dark green and wet. None of these leaf injuries result in the appearance of “dirty” leaf undersides.

Figure 2. Downy mildew ‘look-alike’ symptoms on cucumber.

Dr. Hausbeck’s work is funded in part by MSU‘s AgBioResearch.

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