Forecasting Sclerotinia Disease on Lettuce: Toward Developing a Prediction Model for Carpogenic Germination of Sclerotia

Abstract

The feasibility of developing a forecasting system for carpogenic germination of Sclerotinia sclerotiorum sclerotia was investigated in the laboratory by determining key relationships among temperature, soil water potential, and carpogenic germination for sclerotia of two S. sclerotiorum isolates. Germination of multiple burials of sclerotia to produce apothecia also was assessed in the field with concurrent recording of environmental data to examine patterns of germination under different fluctuating conditions. Carpogenic germination of sclerotia occurred between 5 and 25 degrees C but only for soil water potentials of >/=-100 kPa for both S. sclerotiorum isolates. Little or no germination occurred at 26 or 29 degrees C. At optimum temperatures of 15 to 20 degrees C, sclerotia buried in soil and placed in illuminated growth cabinets produced stipes after 20 to 27 days and apothecia after 27 to 34 days. Temperature, therefore, had a significant effect on both the rate of germination of sclerotia and the final number germinated. Rate of germination was correlated positively with temperature and final number of sclerotia germinated was related to temperature according to a probit model. Thermal time analysis of field data with constraints for temperature and water potential showed that the mean degree days to 10% germination of sclerotia in 2000 and 2001 was 285 and 279, respecttively, and generally was a good predictor of the observed appearance of apothecia. Neither thermal time nor relationships established in the laboratory could account for a decline in final percentage of germination for sclerotia buried from mid-May compared with earlier burials. Exposure to high temperatures may explain this effect. This, and other factors, require investigation before relationships derived in the laboratory or thermal time can be incorporated into a forecasting system for carpogenic germination.

Carpogenic Germination and Viability Studies of Sclerotia of Sclerotinia sclerotiorum Causing Lettuce Drop

“…Abdullah et al, (2008) found that sclerotia did not grow after 7 days at 30 and 35 o C, however, when these sclerotia were incubated at 25 o C, normal myceliogenic growth resumed. Sclerotia that were exposed to 40 and 45 o C failed to grow after 21 days at 25 o C. The upper thresholds temperature for conditioning and germination of sclerotia were 20 and 25°C, respectively (Clarkson et al, 2007). The sclerotial germination of S. sclerotiorum tended to decrease as soil temperature increased from 15 to 40°C, with no germination observed at 40°C after 1 and 2 weeks, respectively (Matheron and Porchas, 2005).…”

Biological control of the soil-borne fungal pathogen Sclerotinia sclerotiorum –– a review

“…No apothecia formation was observed below 70 to 80% saturation. Clarkson et al (2004) showed that carpogenic germination of sclerotia occurred between 5 and 25 o C, but only when the soil water potential was >-100kPa. Above 26°C no apothecia were produced.…”

“…Above 26°C no apothecia were produced. In formed apothecia, the maturation of asci takes about 72-84 h (Clarkson et al 2004). Ascospore maturation is a complex process that is influenced by multiple factors.…”

“…Whetzelina sclerotiorum (Lib) Korf and Dumont 1972 (phylum Ascomycota)] is an ubiquitous pathogen of many plants belonging to the families Solanaceae, Cruciferae, Umbelliferae, Composite, Chenopodiaceae and Leguminosae (Kohn 1979;Willets and Wong 1980;Boland and Hall 1994), which was first reported from sunflower in 1861 (Purdy 1979). The fungus infects leaves, flowers, fruits and stems of the host plants, inducing diseases that can develop during the vegetation period or at the post-harvest stage, and cause severe losses to economically important crops in temperate regions of the world, mainly bean, carrot, pea, lettuce, mustard, canola, lentil and sunflower Clarkson et al 2004;Del Rio et al 2007). For example, in one major rapeseed cultivation area of China, S. sclerotiorum was reported to infect almost 4 to 7 million ha annually (Ni et al 2014).…”

Sclerotinia sclerotiorum disease prediction: A review and potential applications in South Africa

“…2 The interaction between RH and air temperature is pivotal to the formation of apothecia, while soil temperature and moisture influence sclerotial germination responses. 1,11 Atmospheric temperatures ranging from 5 °C to 25 °C with high RH (not quantified in the literature cited) for approximately 10 days promote the development of apothecia. 2,18 When RH is at an optimum and air temperatures are ≥30 °C or ≤5 °C, increased incubation time and reduced carpogenic germination rate have been recorded, despite optimum RH.…”