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Photo: Pecan husks displaying black lesions caused by a fungus are shown beside a photo of pecan nuts.
ARS researchers have identified compounds from extracts of bacteria that live inside beneficial nematodes that can suppress pecan scab (shown on left), a major fungal disease affecting pecan production in the southeastern United States.

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Compound from Bacteria Could Be Useful Against Pecan Scab

By Sharon Durham
September 17, 2014

Bacteria that live inside the guts of tiny nematodes could hold the key to controlling pecan scab, a major fungal disease that affects pecan production in the southeastern United States.

U.S. Department of Agriculture (USDA) scientists discovered nematode-dwelling bacteria that produce chemical compounds that control the fungus Fusicladium effusum, which causes pecan scab.

Agricultural Research Service (ARS) plant pathologist Clive Bock, entomologist David Shapiro-Ilan, chemist Charles Cantrell, and plant pathologist David Wedge examined chemical extracts of the bacteria to identify the major components responsible for suppressing pecan scab. ARS is the USDA’s chief intramural scientific research agency, and this research supports the USDA priority of promoting international food security.

Bock and Shapiro-Ilan work at the ARS Fruit and Tree Nut Research Laboratory in Byron, Georgia. Cantrell and Wedge work at the ARS Natural Products Utilization Research Unit in Oxford, Mississippi.

The bacteria, according to Shapiro-Ilan, live in the guts of beneficial nematodes in the genera Steinernema and Heterorhabditis. The bacteria are critical in helping the beneficial nematodes kill their insect hosts, and can be grown in petri dishes. Extracts of the cultures contain antimicrobial metabolites that are active against a wide range of microbial pathogens of animals and plants, including bacteria and fungi.

The extract found to be most toxic to the pecan scab fungus was purified and found to contain trans-cinnamic acid. Laboratory test results showed that trans-cinnamic acid was toxic to the pecan scab fungus in amounts as low as 148-200 micrograms per milliliter in solid culture and 64 micrograms per milliliter in liquid culture.

Conventional chemical fungicides have been widely used to control pecan scab, but in some growing seasons, more than 10 sprays are required to ensure adequate control of the disease on susceptible pecan cultivars. As a result, F. effusum has now developed resistance to at least two classes of fungicide, according to Bock.

This work was published in the Journal of Pest Science in March 2014.

Read more about this research in the September 2014 issue of Agricultural Research magazine.