Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/11/2009
Publication Date: 6/1/2009
Citation: Bonde, M.R., Nester, S.E., Moore, W.F., Allen, Jr, T.W. 2009. Comparative susceptibility of kudzu accessions from Southeastern United States to infection by Phakopsora pachyrhizi. Plant Disease. 93(6):593-598. Interpretive Summary: Soybean rust, caused by a fungal plant pathogen known in Asia for more than 100 years, was discovered for the first time in the U.S., in 2004. Since then, soybean rust has caused only minor economic losses. Potential losses in the U.S., in the future, hinge on whether or not the pathogen can survive winters in appreciable amounts in the absence of soybean. In an earlier paper, we showed that kudzu, a serious weed in Southeastern U.S. estimated to infest 2 to 7 million acres, can be highly susceptible to soybean rust. It potentially could act as an overwintering bridge for the disease. The present paper compares the susceptibility of kudzu accessions from several locations in Southeastern U.S. to three isolates of the rust fungus. Approximately half of the kudzu accessions were highly susceptible and half highly resistant. The results suggest that kudzu could play a major role in survival of the pathogen in some areas of the country. Susceptibility of kudzu at specific locations could indicate where the disease may eventually be most serious.
Technical Abstract: Soybean rust, caused by Phakopsora pachyrhizi, was first discovered in the United States (U.S.), in the fall of 2004. The potential for economic loss in the U.S. hinges largely on whether or not the pathogen can readily survive winters in the absence of soybean. Kudzu (Pueraria montana var. lobata) is known to become infected by P. pachyrhizi in Asia and South America, and is widely distributed in the southeastern U.S. This study examined reactions of kudzu accessions from several areas of southeastern U.S. to three isolates of P. pachyrhizi, one each from Alabama, Louisiana, and Brazil. Susceptible tan (Tan) lesions and resistant reddish-brown (RB) lesions, previously described on soybean, also were found on kudzu. However, in contrast to soybean, the RB reaction on kudzu was very common, with approximately 50 percent frequency. Tan lesions averaged 3.3 uredinia per lesion with an average diameter per uredinium of 122 um. RB lesions had an average of 0.3 uredinia per lesion with an average uredinial diameter of 75 um. Uredinial numbers and diameters could be used to measure susceptibility of kudzu to soybean rust. In 19 of 25 instances in which we sampled multiple kudzu plants grown from seed from an individual site, each plant gave the same reaction, namely all Tan or RB, to the individual pathogen isolates. This suggested that plants from a specific site tended to be genetically identical with respect to rust reaction, a result of vegetative propagation. Only six of 85 plants produced a different reaction to one isolate as compared to the other isolates. The existence of physiological specialization for P. pacyyrhizi on kudzu, concomitant with a tendency for kudzu plants to react similarly at individual sites to all isolates, could explain why soybean rust on kudzu is common in some areas, but absent in others.