Environmental Sensitivity of Soybean Cultivar Response to Sclerotinia sclerotiorum

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Abstract
The sporadic occurrence of Sclerotinia stem rot in soybeans often is attributed to the sensitivity of Sclerotinia sclerotiorum to environmental factors. Environmental sensitivity in soybean response to the pathogen also could contribute to the unpredictable nature of this disease. We used stability analysis to determine whether soybean cultivar response to S. sclerotiorum was sensitive to light and temperature. Five greenhouse experiments examined the response of seven cultivars to limited-term inoculation with S. sclerotiorum. The cultivars, selected at random from Pennsylvania variety trials, represented maturity groups grown in Pennsylvania and other states sharing that latitude. Photon flux density of photosynthetically active radiation (PAR) and temperature were recorded hourly and varied among experiments. Environmental sensitivity was detected in the response of five cultivars to S. sclerotiorum when individual cultivar disease ratings (assessed 6 days after inoculation) were regressed against the mean disease rating of each experiment. Stability analysis with temperature during the 48-h inoculation period as the environmental index found that all cultivars responded similarly to the number of hours that temperatures were 22 degrees C. In contrast, cultivars separated into PAR-sensitive and PAR-insensitive groups when the environmental index was moles of PAR at a photon flux density >/=475 mumol m(-2) s(-1) during the inoculation period. The photon flux density of PAR on a cloudy day in the field is </=200 mumol m(-2) s(-1). The two cultivar groups were identical to those identified as environmentally sensitive and insensitive in the initial analysis. The PAR-sensitive cultivars had homogeneous negative regression coefficients different (P = 0.001) from zero. As the accumulated moles of PAR during the inoculation period increased, the disease ratings of the PAR-sensitive cultivars decreased; the disease ratings of the PAR-insensitive cultivars remained stable. The photon flux density of PAR during the 48-h inoculation period explained the environmental sensitivity of soybean cultivar response to S. sclerotiorum in these experiments.