PATHOGENICITY OF BAR-TRANSFORMED COLLETOTRICHUM-GLOEOSPORIOIDES F SP AESCHYNOMENE

Colletotrichum gloeosporioides f. sp. aeschynomene was transformed wit h a gene (bar) for resistance to bialaphos (a natural herbicide) and e valuated for pathogenicity and virulence. A C.g.aeschynomene transform ant (48-5b) containing the bar gene was stable and resistant to bialap hos up to a concentration of 100 mu g/ml. Plants were stem-inoculated for pathogenicity tests and foliar-sprayed for virulence tests with th e wild-type or the 48-5b isolate at rates of 0 to 2.5 x 10(10) spores/ m(2) with bialaphos at 0 and 0.56 kg a.i./ha, The host range of C.g.ae schynomene was not altered by the transformation, Disease severity on northern jointvetch (Aeschynomene virginica) was similar for treatment s with the wild-type and transformant isolates without bialaphos, with 90% or greater disease severity occurring at 2.5 x 10(7) spores/m(2). There was no significant difference in disease severity on northern j ointvetch between treatments of the transformed isolate with or withou t bialaphos, At 2.5 x 10(6) spores/m(2), however, the coapplication of the transformant with bialaphos resulted in a higher level of disease severity than that of the wild-type isolate without bialaphos. Diseas e severity on northern jointvetch from the treatment of wild-type isol ate with bialaphos was significantly less than that of the transforman t with bialaphos at spore concentrations of 2.5 x 10(6) to 2.5 x 10(8) sores/m(2), On Indian jointvetch (Aeschynomene indica), the coapplica tion of bialaphos and C.g.aeschynomene resulted in a significant syner gistic effect (increase in disease severity) with the transformant and an antagonistic effect (decrease in disease severity) with the wild-t ype, The results demonstrate that at recommended rates of C.g.aeschyno mene for the control of northern jointvetch, there is no benefit from coapplying the fungus with subleathal levels of bialaphos. However, at spore concentrations lower than that recommended, significantly great er disease development can occur when sublethal levels of bialaphos ar e applied with a bialaphos-resistant isolate. Furthermore, the coappli cation of such a resistant isolate with sublethal levels of bialaphos can significantly extend the control range of this fungus toward India n jointvetch. This suggests that improvements in the virulence and con trol range of a mycoherbicide may be realized if it were altered to en dogenously produce a nonselective phytotoxin such as bialaphos. (C) 19 96 Academic Press, Inc.