Margaret Tuttle McGrath
Department of Plant Pathology and Plant-Microbe Biology, Cornell University
Long Island Horticultural Research and Extension Center
3059 Sound Avenue, Riverhead, NY 11901; email@example.com
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For the first time in NY, symptoms were observed at a commercial farm of pod rot in snap bean caused by the pathogen that causes Phytophthora blight in other crops. This happened on Long Island in 2008. Phytophthora blight has been occurring routinely for many years, sometimes causing extensive losses, on peppers and cucurbits on LI. Occasionally eggplant and tomato are affected. The disease is also common on these crops in many other areas in the US, including upstate NY.
|Fig 1. Phytophthora pod rot.|
|Fig 2. Phytophthora capsici.
Photo Courtesy of Jadwiga
LIHREC, Cornell University.
Affected areas of pods had the characteristic white, yeast-like growth of the pathogen (fig 1). As is the case with other susceptible crops, this growth when examined under a microscope was observed to be spores (fig 2). The incredibly vast quantity of spores on affected tissue can account for the explosive ability of blight to develop. Affected areas were dark and water-soaked. More severely affected pods were shriveled.
Prior to this sighting on LI, snap beans had only been observed affected by Phytophthora blight in MI. The first report of this disease on snap beans there was in 2003. It has been causing losses in MI ever since. Large, water-soaked lesions similar to what the blight pathogen (Phytophthora capsici) causes on other susceptible crops as well as plant collapse have been observed in addition to pod rot. The only other occurrence of Phytophthora capsici affecting a legume crop has been lima bean in DE and MD where pod rot caused by this pathogen was first observed in 2000.
Until now there have been no other reports of blight affecting snap beans in other states. This combined with the fact that Phytophthora blight had been occurring on other crops in MI for many years before snap beans started to be affected, led to the speculation that a new strain able to infect snap bean might have developed there. However, based on the circumstances with the LI occurrence, favorability of environmental conditions as well as availability of inoculum are important factors that may not have occurred previously. Environmental conditions in 2008 were very favorable for Phytophthora blight. Additionally, the affected bean crop was produced in a low section of a field where Phytophthora blight occurred on pepper the previous year. About 3 inches of rain fell over 2 days starting 3 days before the beans were harvested. Symptoms developed overnight in the harvest bins. Another indication of the favorability of conditions for blight in early September 2008 when this occurred is the high incidence of tomato fruit rot due to Phytophthora.
An investigation was
undertaken after pod rot occurred on LI to determine if it was caused by a
unique strain of the pathogen. Under the controlled conditions of a
laboratory, bean pods were successfully infected with Phytophthora spores
from squash and tomato, as well as bean (fig 3). Additionally, a
collection of isolates of Phytophthora
capsici taken from the
beans were found to be similar to a collection of isolates obtained at the same
time from a cucurbit crop on the same farm. They were similar in terms of
resistance to the fungicide Ridomil and presence of pathogen mating types.
These observations suggest the strain on bean is not new and unique, as was
thought to be a possible reason that beans had not previously been observed
affected by Phytophthora capsici outside of MI.
|4 days after inoculation.||9 days after inoculation.|
|Fig 3. Symptoms of Phytophthora pod rot developing on pods after placing on them agar plugs containing P. capsici isolated from snap bean (top) and from tomato (left). Pods with plugs lacking the pathogen (right) are asymptomatic.|
Growers producing beans who have had Phytophthora blight develop on other crops on their farm are advised to now treat beans as a susceptible crop rather than a suitable rotation crop. Cultural practices are the only management options for blight in beans currently. Avoid planting beans in low areas especially in fields where blight has developed previously. Implement practices to improve soil drainage. Unfortunately none of the fungicides recommended for managing Phytophthora blight in other crops are labeled for use in beans. A fungicide efficacy experiment will be conducted at LIHREC to obtain data to support label expansion for some of these fungicides. All bean types are considered at risk based on the fact that all 12 types of beans tested in experiments conducted in MI were found to be susceptible to strains of the pathogen that were also able to infect cucumber.
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