APS Homepage

POSTERS: Chemical control

Possible synergistic effect of prohexadione-calcium and acibenzolar-S-methyl in managing shoot blight of apple
Suzanne Slack - Michigan State University. George Sundin- Michigan State University, Cory Outwater- Michigan State University

Fire blight, an economically important disease of pome fruit, has multiple stages of infection including shoot blight. The causal agent, Erwinia amylovora, can infect apple shoots from internal movement via infected flowers or from external infection via bacterial cells disseminated by wind and rain. Young apple trees are susceptible to fire blight infection, and in combination with high-density orchards, shoot blight management is a growing concern. Two products, prohexadione-calcium (Pro-Ca) and acibenzolar-S-methyl (ASM), are used to reduce shoot blight on older trees. However, Pro-Ca causes shoot stunting, which is detrimental for growing trees. This study seeks to determine if low rates of Pro-Ca and ASM can reduce incidence of shoot blight without shoot stunting. During shoot growth, two young ‘Gala’ apple plantings were treated with low rates of Pro-Ca and ASM. Rates of Pro-Ca ranged from .07-.56 kg/ha and ASM at .07-.14 kg/ha; both as stand-alone and tandem treatments. Each experiment was inoculated with E. amylovora strain Ea110 to ensure disease development. Shoots were measured weekly for growth and lesion length after inoculation. Scanning electron microscopy was utilized to measure plant cell wall widths. Disease incidence with Pro-Ca at .14 kg/ha + ASM at .07 kg/ha was significantly lower than with either product alone. Lesion length was similar with a high rate of Pro-Ca alone, but with no significant impact on shoot length. Our results suggest that low rates of Pro-Ca and ASM may work synergistically to reduce incidence of shoot blight on young apple trees.