Vegetable Disease Updates – August 12, 2011

Bob Mulrooney, Extension Plant Pathologist;

Sweet Corn
With a return to some cooler weather be on the lookout for corn leaf rust on sweet corn. Scout the plantings and if you see rust on plants at the whorl stage or younger, rust could become an issue if the hybrid is not resistant to rust. Rust, when heavy, can affect plant health and reduce ear size. The best control is to plant resistant hybrids, but the strobilurin or triazole fungicide work well. On fresh market corn rust on the husks makes ears unsightly to consumers.

Cucurbit downy mildew
was recently observed on cantaloupe in the sentinel plot in Newark. These lesions resemble the same symptoms as seen on cucumber but spore production appears to be very sparse. It has not moved to any pumpkin, winter squash or watermelon so far. The susceptible cucumber in the plot is almost totally defoliated.

Lima Beans
So far weather conditions have not been favorable for downy mildew. It looks like the weather may be changing and getting a bit cooler with more dew and possibly fog in the early morning hours. If it should start raining soon growers and crop consultants should be scouting for downy mildew. Race F of Phythophthora phaseoli was the only race identified in 2006, 2008, 2009, and 2010. Preventative applications of 2 lbs fixed copper, 2 lbs Ridomil Gold/Copper, or 3- 4 pts Prophyte have provided control of downy mildew in the past. The newest formulation of fixed copper from DuPont is Kocide 3000 and it performs as well as the other formulations of copper at the rate of 1.3 lbs/A. The best controls continue to be Ridomil/Gold Copper, Prophyte, or other labeled phosphonate fungicides, and Omega, especially when disease pressure is high. Application at flowering or when pods are first forming is recommended if weather is favorable for disease. If disease is present Ridomil/Gold Copper and phosphonate fungicides have shown to provide some curative activity if applied when downy mildew is first seen. If downy is present in the field do not use copper fungicides alone for curative control, they will not provide control. Another product that is labeled on lima beans for white mold control is Omega but not downy mildew, but in DE this would be a 2ee use that someone like me can recommend since the fungicide is labeled on lima beans. I have three years data that show excellent control of downy mildew at 5.5 fl oz and 8.0 fl oz/A as a preventative application (before disease is found in the field). Omega is not labeled for aerial application, however. Headline from BASF is also labeled for downy mildew. I have tested it and it has provided good control of downy when applied on a 10-day schedule at 6.0 fl oz /A. It does not give as good disease control as Ridomil Gold/Copper or the phosphonates preventatively but the yields have been comparable. It is also labeled for anthracnose which the other products do not control.

Downy mildew caused by Phytophthora phaseoli

Downy mildew on raceme and petiole

Phytophthora capsici on lima bean pod.

Phytophthora capsici will infect lima bean pods as well and can look very similar to downy mildew. P. capsici or lima bean pod rot is usually found in wet low spots in the field. The fungus growth looks more granulated or “pebbly” than downy mildew, microscopic confirmation is encouraged.

Downy mildew on the upper pod and lima bean pod rot on the lower pod. Note the granular appearance of the fungus on the lower pod and the lack of a reddish brown border on the pod infected with lima bean pod rot or Phytophthora capsici.

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