Horticulture Research International, Wellesbourne, Warwick, Warwickshire, CV35 9EF, UK

Background and Objectives
The pathogen Botrytis squamosa causes leaf blight of bulb and salad onions and is a major disease in the UK. The fungus survives over winter as sclerotia in the soil and, when brought to the surface, these provide primary inoculum in the spring by germinating to produce conidia. Once infected, plants quickly develop characteristic yellow sunken lesions and leaves die back prematurely. Further development and sporulation by the pathogen then occurs on the dead leaves. In salad onions there is therefore loss of quality due to leaf spotting and grade size is reduced in bulb onions. A number of models have been put forward to time fungicide sprays for the control of leaf blight. However, none of these consider inoculum from sclerotia and there is little information on the conditions required for the germination and production of conidia. The aim of this work was to assess the effect of temperature and other environmental parameters on sclerotial germination.

Materials and methods
Sclerotia of B. ;squamosa were produced by inoculating a sterile mixture of sand and maizemeal with conidia and incubating for 4-6 ;weeks at 20C. Sclerotia obtained were then retrieved by sieving, dried in an airflow cabinet for 2 ;days and were then stored at 5C for use in experiments. To assess the effect of temperature on germination, sclerotia were placed on moist filter paper in 5-cm Petri dishes and placed at 5, 10,15, 20, 25 and 30C under UV lights and the production of conidia assessed over time. Similarly, the effect of relative humidity was tested by placing sclerotia over solutions of glycerol. To simulate the effect of soil moisture, sclerotia were also placed on filter paper moistened with solutions of PEG to give different water potentials. To assess germination of sclerotia in the field, four mesh bags containing 40 sclerotia were placed in the field every week. Bags were removed after 1, 2, 3 or 4 ;weeks and sclerotia observed for germination and the process repeated throughout the year. Rainfall, temperature, relative humidity and soil moisture were measured using a Delta-T logger. The importance of sclerotia as primary inoculum was also assessed in the field by inoculating bulb onion plots with different numbers of sclerotia and assessing subsequent leaf blight development.

Results and conclusions
The rate of conidial production by germinating sclerotia was optimal at 15C. At 5, 10 and 15C there was 100% germination but this declined to only 15% at 25C. The greatest numbers of conidia were produced at 5 and 10C and was much reduced at 25C. No condia were produced at 30C. Sclerotia did not germinate at relative humidities below 98% and conidial production was highest at 100% r.h. Decreasing water potential had the effect of reducing sclerotial germination rates at suboptimal temperatures and also resulted in fewer conidia being produced. However, some conidia were still produced at potentials as low as -1.2 ;MPa. A temperature model for conidial production from sclerotia was produced. Sclerotia placed in the field germinated according to the temperature model derived from in vitro work, except when soil moisture became limiting. Incorporating soil moisture content into a model for production of conidia is in progress. The bulb onion plot inoculated with the higher number of sclerotia developed leaf blight more rapidly and resulted in greater severity of disease. This is the first time that sclerotia of B. ;squamosa have been used as inoculum in the field and demonstrates that they can be an important source of infection. A model for the production of conidia based on temperature and soil moisture will be useful in identifying when inoculum is first produced in the field and hence add to existing forecasting systems to aid spray timing decisions.