The epidemiology of tomato mosaic X. Persistence of TMV‐infected debris in soil, and the effects of soil partial sterilization

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Abstract
SUMMARY: The length of exposure necessary to inactivate tobacco mosaic virus (TMV) at different temperatures depends on the concentration of virus and its location, whether in sap or leaf and root debris. However, even when eliminated from the upper 30 cm. of border soil, using standard commercial steaming equipment, debris at lower depths remained infective for at least 22 months in fallow soil. Steaming as usually done commercially in Britain may not eradicate TMV, even from the upper soil layer, because an adequate temperature is not maintained long enough to penetrate thick roots or to reach every part of the soil, especially that near brickwork, water‐pipes or other structures. Hoddesdon pipes were much more effective for this purpose than spiked steam‐pipes.TMV in undiluted tomato leaf sap was inactivated in one experiment in 20 min., but not in 10 min., at 82°C., and in 5 min. at 85 or 88°C.; in another experiment, elimination took between 10 and 20 min. at 85° and 88°C. In dead root tissue TMV was inactivated by moist heat within 10 min. at 85°C., but in fairly fresh moribund roots it took at least 20 min. at 90°C. TMV was not inactivated within dry debris in dry heat even after 20 min. at 100°C.TMV in tomato root debris was inactivated by gamma irradiation of two million rads, and partially inactivated by exposure to daylight for 12 weeks.The early incidence of mosaic in tomato crops grown in TMV‐contaminated soils treated with chloropicrin or metham‐sodium was often greater than on plants in untreated soils. Chloropicrin, metham‐sodium methyl isothiocyanate (MIT), D‐D and nabam, tested at concentrations up to five times greater than those expected to occur in soil using recommended doses, never inactivated TMV either in root debris or when mixed with expressed leaf sap, but sometimes increased its persistence. The mean effect of treating buried roots with chloropicrin and metham‐sodium was to increase the proportion of infective root fragments, 4–13 months after treatment, from 33% in the untreated controls to 61% and 73% respectively. Formaldehyde and cresylic acid almost eliminated TMV in sap and roots when applied at five times the normal rate, and at the normal rate formaldehyde decreased TMV concentration considerably. Soil fumigation with methyl bromide was ineffective under glasshouse conditions.The microflora and microfauna of glasshouse soils were surveyed before and after partial sterilization. The different treatments significantly affected the micro‐organisms differently, but no causal relation was established between them and the persistence of TMV. Species of Bacillus, Corynebacterium and Pseudomonas were obtained from decaying roots regardless of treatment. Isolates of Chaetomium spp. formed a greater proportion of the fungus cultures from chloropicrin‐treated roots than from roots treated in other ways. Sepedonium byssicola was commonly isolated only from roots treated with steam or MIT, and Doratomyces spp. only from roots stored in MIT‐treated soils. Other isolates included species of Botryotrichum, Fusarium, Mortierella, Penicillium and Pyrenochaeta.

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