Effect of soil microorganisms on viability of sclerotia of Ciborinia camelliae, the causal agent of camellia flower blight

Abstract

Two isolates each of Trichoderma virens and Coniothyrium minitans, four of T. longipile, and one each of T. tomentosum and Clonostachys rosea, all known to be sclerotial parasites of Sclerotinia sclerotiorum, were tested for their antagonism to sclerotia of the closely related pathogen, Ciborinia camelliae. Treatment of sclerotia in sterile sand or potting‐mix assays with isolate suspensions of 106 or 107 spores/ml had little effect on the viability of sclerotia, 12–37 weeks after treatment. In a field trial, spore suspensions of T. virens LU556, C. rosea LU155, and C. minitans LU1 12, sprayed in July, August, and September, at a total of 0.8–2.9 × 1011 spores/m2 onto soil naturally infested with over 600 sclerotia/m2, had no effect on apothecial production or on sclerotial numbers recovered at the end of the trial, 8 weeks after the last mycoparasite application. Parasitism of sclerotia by C. minitans, C. rosea, and T. virens was not observed under scanning electron microscopy examination. Thus, the test isolates were not considered effective parasites of C. camelliae sclerotia. Subsequently, 376 microorganisms isolated from C. camelliae sclerotia baits that had been buried in soil, and 30 microorganisms from decaying C. camelliae sclerotia, were screened in vitro for their ability to parasitise healthy sclerotia of the pathogen. The most effective antagonists were T. virens LU569 and T. viride LU570, which reduced numbers of viable C. camelliae sclerotia after 14 weeks by 53% and 43%, respectively. These isolates warrant further evaluation under field conditions where one can potentially manipulate the soil environment by the addition of soil conditioners to enhance their efficacy in situ.