October 7, 2000
A new kind of soil bacteria found living around sugar beet roots may offer a natural defense against fungi that menace the crop. Grown on 1.5 million acres, sugar beets supply roughly 50 percent of the nation's sucrose.
Agricultural Research Service microbiologist David Kuykendall is investigating the bacteria's potential as a fungicide alternative in studies at the ARS Molecular Plant Pathology Laboratory (MPPL) in Beltsville, Md. In early findings from a series of petri dish experiments, colonies of two Pseudomonas bacteria exuded substances that curbed the growth of Cercospora beticola, a fungal rival that lives on the plant's leaves.
Cercospora causes a rapid, progressive disease called leaf spot that can defoliate susceptible sugar beet cultivars. Fueled by humid conditions, severe outbreaks can reduce sucrose yields by more than 30 percent. Growers sometimes spray chemical fungicides when spots cover 3 percent of the leaves' surface. With fungicide use, however, comes legitimate concern over the chemicals' effectiveness due to the emergence of new, fungicide-tolerant Cercospora strains, notes Kuykendall.
As an alternative, he envisions spraying the soil bacteria directly onto the crop's leaves when conditions are ripe for Cercospora outbreaks. The Pseudomonas bacteria, two strains labeled ND6-2 and ND9L, were obtained by ARS researchers Garry Smith and John Eide in North Dakota from the rhizosphere, or soils around sugar beet roots.
Now, Kuykendall is conducting plant host studies to learn whether either strain poses any danger to other crops. By analyzing their DNA sequences, for example, Kuykendall has identified a high degree of genetic similarity to another Pseudomonas bacterium, P. corrugata, a tomato plant pathogen.
Results confirming the bacterial strains' virulence would probably rule out their use as a foliar spray. In that event, Kuykendall plans an alternative strategy: isolate specific genes that give rise to their anti- fungal secretions, and insert the bacterial genes into sugar beet. He described the new findings in a recent issue of Sugar Journal.
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