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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 <19 degrees C, 19 to 22 degrees C, or >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

Drought stress in the field develops gradually, allowing plants to acclimate to the stress, however, in the greenhouse it often develops too rapidly to allow acclimation. A technique was developed to induce drought stress on alfalfa (Medicago sativa L.) in the greenhouse, in a manner conducive to plant acclimation. The technique allows: (i) gradual development of drought stress; (ii) use of plants with large root systems; and (iii) imposition of several cycles of drought. The method uses 90‐cm‐tall by 20‐cm‐diam. containers and a growth medium that allows development of plant drought stress within the working range of soil tensiometers. The growth medium, a 2:1 mixture, by volume, of commercial potting mix and coarse sand, is amended with gypsum and slow‐release nutrients. Drought‐stressed plants were shorter than nonstressed plants and had lower leaf and stem dry weights, total aerial biomass, leaf water potentials, stomatal conductances, and osmotic potentials. These responses are indicative of plants undergoing drought acclimation. The technique is an inexpensive, low maintenance method of inducing a gradually intensifying drought stress in the greenhouse.

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