Pathogenicity ofbar-TransformedColletotrichum gloeosporioidesf. sp.aeschynomene

Colletotrichum gloeosporioidesf. sp.aeschynomenewas transformed with a gene (bar) for resistance to bialaphos (a natural herbicide) and evaluated for pathogenicity and virulence. AC.g.aeschynomenetransformant (48-5b) containing thebargene was stable and resistant to bialaphos up to a concentration of 100 μg/ml. Plants were stem-inoculated for pathogenicity tests and foliar-sprayed for virulence tests with the wild-type or the 48-5b isolate at rates of 0 to 2.5 × 1010spores/m2with bialaphos at 0 and 0.56 kg a.i./ha. The host range ofC.g.aeschynomenewas not altered by the transformation. Disease severity on northern jointvetch (Aeschynomene virginica) was similar for treatments with the wild-type and transformant isolates without bialaphos, with 90% or greater disease severity occurring at 2.5 × 107spores/m2. There was no significant difference in disease severity on northern jointvetch between treatments of the transformed isolate with or without bialaphos. At 2.5 × 106spores/m2, 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. Disease severity on northern jointvetch from the treatment of wild-type isolate with bialaphos was significantly less than that of the transformant with bialaphos at spore concentrations of 2.5 × 106to 2.5 × 108sores/m2. On Indian jointvetch (Aeschynomene indica), the coapplication of bialaphos andC.g.aeschynomeneresulted in a significant synergistic effect (increase in disease severity) with the transformant and an antagonistic effect (decrease in disease severity) with the wild-type. The results demonstrate that at recommended rates ofC.g.aeschynomenefor 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 greater disease development can occur when sublethal levels of bialaphos are applied with a bialaphos-resistant isolate. Furthermore, the coapplication of such a resistant isolate with sublethal levels of bialaphos can significantly extend the control range of this fungus toward Indian jointvetch. This suggests that improvements in the virulence and control range of a mycoherbicide may be realized if it were altered to endogenously produce a nonselective phytotoxin such as bialaphos.