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New cowpea diseases in 2018 – white mold and phomopsis stem canker

New cowpea diseases in 2018 – white mold and phomopsis stem canker
Examples of white mold and stem canker: TOP: Bleaching of stems and branches in cowpea plants affected by white mold. BOTTOM LEFT: Brown lesions on cowpea stems characteristic of Phomopsis stem canker. BOTTOM RIGHT: Lodging of stem canker-infected sunflower plants in the field. Courtesy Photo

New pulse crops such as chickpeas (garbanzo beans), cowpeas and dry yellow peas are being produced in Nebraska, and the interest among growers continues to increase. The acreage planted to these crops is not as high as dry beans, but is still significant.

A pulse crop is a legume that is grown and utilized as a mature, dried seed, which also includes the dry bean – a crop widely cultivated commercially in Nebraska for a century or more.

One of the newest of these crops is the cowpea, also known as the black-eyed pea. For the past two years, I have been searching for potentially problematic disease issues within fields planted to cowpeas. Last fall I wrote an article describing the disease that was the most predominant from all sites surveyed during 2017-18. At this point, it appears to be the same bacterial wilt disease caused by Curtobacterium flaccumfaciens that we see commonly in dry bean production in Nebraska.

Similar foliar symptoms consisting of wavy, interveinal yellowing with necrotic borders and wilting of affected plants were observed, which are all characteristics of the disease in dry beans. We also confirmed that after isolation these cowpea pathogens caused disease on both cowpea and dry beans after greenhouse inoculations.

Within some of our research plots at the Panhandle Research and Extension Center in 2018, we also observed two additional fungal diseases that routinely occur in other crops grown in western Nebraska.  However, they were somewhat unexpected on cowpeas because they have apparently rarely if never been reported causing disease in this crop.

 White Mold

The white mold disease is familiar to anyone who has grown dry beans in Nebraska. It is caused by a fungus (Sclerotina sclerotium) that also will attack other Panhandle crops like sunflowers and potatoes.  Its presence was not noted until harvest, when the very conspicuous bleached white plants (See top photo) were readily seen after leaf drop.

 Phomopsis Stem Canker

After closer inspection, we also noticed light- brown to tan, oval-shaped lesions at the nodes (point of attachment of petioles) on the main stems (Bottom left photo). These symptoms are consistent with another fungal disease of sunflowers that has increased in both distribution and severity in this region over the last five years, referred to as Phomopsis stem canker.

The pathogen initially attacks leaves and moves down the petioles (structure that attaches the leaves to stems), where it invades the stalks. This causes a rot and the plants to lodge at the point of infection, resulting in yield reduction when fully developed heads fall to the ground (Bottom right photo) and are not picked up by harvesting equipment.

Fungal isolates retrieved from the symptomatic lesions on cowpea stems were identified as Diaporthe (formerly Phomopsis) gulyae, an extremely virulent species of this pathogen that has recently been discovered in Nebraska sunflower fields. Although both S. sclerotiorum and D. gulyae are well-known as pathogens of legume crops, particularly in soybeans, it was somewhat surprising to discover them affecting cowpeas. These findings are novel and will be new disease reports on this crop when published later this year or early next year.

Conclusions and Future Studies

Fortunately, neither pathogen affected yield results for our studies in 2018, likely due to infection occurring late in the season after pods had already filled. However, this is important information to at least be aware of, if cultivation of this crop is going to continue in rotation with other traditional host crops like sunflowers and dry beans.

We are conducting new experiments in the greenhouse with the causal agents of both diseases obtained in 2018 to formally confirm identities of pathogens and assess their ability to cause disease on all crops involved – sunflowers, dry beans, and cowpeas.

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