Environmental conditions influencing Sclerotinia sclerotiorum infection and disease development in lettuce

Abstract

The environmental factors that influence infection of lettuce by ascospores of Sclerotinia sclerotiorum, and subsequent disease development, were investigated in controlled environment and field conditions. When lettuce plants were inoculated with a suspension of ascospores in water or with dry ascospores and exposed to a range of wetness durations or relative humidities at different temperatures, all plants developed disease but there was no relationship between leaf wetness duration or humidity and percentage of diseased plants. Ascospores started to germinate on lettuce leaves after 2-4 h of continuous leaf wetness at optimum temperatures of 15 -25°C. The rate of development of sclerotinia disease and the final percentage of plants affected after 50 days were greatest at 16 -27°C, with disease symptoms first observed 7-9 days after inoculation, and maximum final disease levels of 96%. At lower temperatures, 8 -11°C, disease was first observed 20 -26 days after inoculation, with maximum final disease levels of 10%. Disease symptoms were always observed first at the stem base. In field-grown lettuce in Norfolk, 2000 and 2001, inoculated with ascospore suspensions, disease occurred only in lettuce planted in May and June, with a range of 20 -49% of plants with disease by 8 weeks after inoculation. In naturally infected field-grown lettuce in Cheshire, 2000, disease occurred mainly in lettuce planted throughout May, with a maximum of 31% lettuce diseased within one planting, but subsequent plantings had little (≤ 4%) or no disease. Lack of disease in the later plantings in both Norfolk and Cheshire could not be attributed to differences in weather factors.

A Model for Sclerotinia sclerotiorum Infection and Disease Development in Lettuce, Based on the Effects of Temperature, Relative Humidity and Ascospore Density

“…Ascospore germination on lettuce leaves was optimum at 15–25°C and the rate of Sclerotinia disease development was greatest at 16–27°C [18]. However, we could not establish a relationship between leaf wetness period, or duration of different RH levels (up to 6 d), and disease levels.…”

“…The mean rate of spore germination was calculated from the data collected by Young et al, [18] (Table 1) under conditions of continuous leaf wetness and fitted using the temperature response function of Yan and Hunt [23]. The percentage rate of germination (h −1 ) was calculated as:…”

“…The model functions related to these phases were parameterised using results from CE experiments carried out in this study and also from previously published data [18].…”

“…This was done using data from previous CE experiments investigating the effect of temperature on spore germination and disease development [18], as well as data from new experiments investigating the effect of spore density and RH on disease development. Once assembled, the model was validated using three independent CE experiments where both temperature and RH were varied.…”

Effect of temperature on mycelial growth of Trichoderma, Sclerotinia minor and S. sclerotiorum, as well as on mycoparasitism

“…Similarly, Imolehin et al (5) showed that sclerotia of S. minor germinated and exhibited mycelial growth at temperatures ranging from 6 to 30 o C with optimum growth at 18 o C and infection of lettuce plants occurred at temperatures ranging from 6 to 24 o C. For S. sclerotiorum, Young et al (14) verified that ascospores could cause bottom rot in lettuce at temperatures ranging from 8 to 27 o C, but the disease occurred more rapidly and was more severe at temperatures ranging from 16 to 27 o C, peaking at 22 o C.…”