Colletotrichum acutatum Simmonds f.sp. pinea Dingley & Gilmour: its persistence in soil and its infectivity of Pinus radiata D. Don seedlings

Abstract

Persistence of Colletotrichum acutatum Simmonds f.sp. pinea Dingley & Gilmour in pine nursery soils and its infectivity of Pinus radiata D. Don seedlings were studied. Sampling procedures were compared in the search of soils for fungal propagules. Those most frequently used were soil dilution plate method, surface-soil dilution plate technique, and plating of debris picked from soil. A selective medium for direct isolation from soil was prepared comprising potato dextrose agar as the basal medium and incorporating benomyl (50 ppm), quintozene (7.5 or 75 ppm), streptomycin sulfate (100 ppm), chloramphenicol (100 ppm) and chlortetracycline HCl (100 ppm). Studies in plots at Auckland, Tokoroa and Rotorua indicated that fungal structures within P. radiata debris persisted for long periods; up to 25 months in the Auckland plots. In vitro, the viability of conidia in natural soil declined very rapidly; more than 50% of conidia were non-viable within 4 weeks of their introduction into soil incubated at 15 and 25 C. Similar experiments conducted at different matric potentials indicated that drier soils (-0.3 or -0.4 bar) had higher levels of viable conidia over periods of time compared to wetter soils of -0.1 bar or at saturation. Segments of pine seedlings introduced to soils into which various concentrations of conidia were previously incorporated showed increasing colonization with increasing conidial concentration at both 15 and 25 C after 4 days. Maximum colonization of segments occurred at conidial concentrations 106 ml-1 to 107 ml-1. Artificially-inoculated segments of pine seedlings were introduced into soil and then these were recovered at different time periods. After two months in soil 100% recovery of the fungus was obtained from all leaf, stem and root segments plated. Recovery levels from all the plated stem and root segments after 8.5 months were never lower than 68%. Recovery levels from leaf segments were lower because many of these had decomposed leaving only the central xylem strands. Conidia-laden membrane filters were introduced into soil. A method of microscopic study used after removal of these from soil enabled easy observation of resultant structures. Appressoria were formed while the fungus was in soil. Other potential survival propagules were thick-walled, darkly-pigmented, short hyphae within pine debris. It is considered that the short individual cells of such hyphae could survive as chlamydospores, and these are believed to be the prime means of long-term survival in soil. Several fumigants were tested for effectiveness in eradication of the fungus from soil. All chemicals tested, namely chloropicrin, methyl bromide, "Di-Trapex" and dazomet significantly reduced viability of conidia and the fungus in pine debris. Weeds from a pine nursery were inoculated with conidia; of these, only Epilobium ciliatum Raf. became infected. Infection of healthy parts was infrequent but saprophytic growth in older parts frequently killed the plant. This species is therefore a potential alternative host, although its importance as an inoculum source for infection of pine seedlings is unknown. Experimentally it was shown that infection could occur by rain splash of conidia onto healthy plants. Infection of P. radiata seedlings was studied at several constant and alternating temperatures. Initial infection was greatest at higher temperatures (ca 24 C); while full symptoms of the disease were best expressed at lower temperatures (12 to 18 C). Inoculated seedlings had significantly reduced shoot and root development at 12, 15, 18 C, but not at 6, 9, 23, 24, 27 C, after 53 days. Increase in height of healthy seedlings was significantly greater than that of inoculated seedlings, after 52 days at 18 C. Pathogenicity on P. radiata seedlings of an isolate of C. acutatum f.sp. pinea isolated from soil, "whitish isolate", was comparable to the normal "coloured isolate".