An old disease in a new crop – cowpea bacterial wilt
Robert M. Harveson, Extension Plant Pathologist, Panhandle R&E Center, Scottsbluff
Currently in Nebraska there is a growing interest in producing new pulse crops. The dry edible bean is one example of a pulse crop, but there are several more at present being grown commercially.
A pulse crop is a legume that is grown and utilized as a mature, dried seed. Other examples include chickpeas (garbanzo beans), lentils, dry yellow peas, and cowpeas.
Compared with dry beans, our knowledge of these new crops is lacking. Therefore we have been identifying and studying diseases as they appear in order to understand how we might attempt to proactively address manage them when needed. Ascochyta blight has been the primary disease observed on chickpeas, while a bacterial disease complex has been observed for dry yellow peas. This complex consists of several species resembling brown spot and common blight in dry beans.
The newest member of this group of pulse crops is the cowpea, also referred to as the black-eyed pea. Despite its name, botanically, it is a type of bean, and not a true pea. Its acreage in Nebraska has not been as high as that of the dried yellow pea, but nevertheless, there is still substantial interest in using it as a rotational crop with dry beans, corn, wheat, and sugar beets.
I have been surveying cowpea fields for the last two years looking for the predominant diseases that may be problematic if the crop continues. From these studies it appears that cowpeas are susceptible to several bacterial diseases, similar to that of dry beans and yellow peas.
Based upon these preliminary observations, I have noted that a disease very similar to bacterial wilt appears to be the most commonly occurring one. Interveinal yellowing with a necrotic border has been observed, which is characteristic of bacterial wilt in dry beans.
The damage was more widespread and severe in fields after severe hail storms. Fields not exposed to storms were only affected by randomly spaced, individual infected plants, and would not have contributed greatly to large yield reductions. To date, infections have been documented from multiple fields and counties throughout the Panhandle, stretching from Kimball to Hemingford, so it was not just an isolated event from one location.
I believe at this point that it is the same pathogen (Curtobacterium flaccumfaciens) that causes the wilt disease in dry beans. We have isolated bacteria with both yellow and orange-pigmented colonies from symptomatic cowpea plants.
We have also inoculated these isolates on both cowpea and dry bean plants in the greenhouse. Resulting symptoms on both were consistent with bacterial wilt, indicating that the isolates are pathogenic on both crops.
However, there were differences in the plant response to infection between the two crops. The symptoms were more severe with dry beans than cowpeas. Symptoms of infection also began later after inoculation in cowpeas than dry beans. Although both crops are apparent hosts, the cowpeas were not as severely affected, suggesting a disease tolerance that is not present in susceptible dry bean cultivars.
To my knowledge, this will be a first report of this disease on cowpeas from natural field infections in the Western Hemisphere. The only other report at all of wilt on cowpeas I am aware of came from Iran in 2015.
Further investigations are ongoing to definitively confirm the identities of these pathogens and continuing to study their potential damage to cowpeas and design methods for management.