The occurrence of a quiescent infection of stone fruits caused by Sclerotinia fructicola (wint.) Rehm.

An infection of immature peach and apricot fruits by Sclerotinia fructicola (Wint.) Rehm. is described. Since infection results in macroscopically visible lesions it is proposed that this type of infection be known as "quiescent" rather than latent. Symptoms have been induced in immature apricots by inoculation with conidia of the fungus and, after a period of quiescence, progressive rots developed from such infections as the fruit ripened. The fungus was re-isolated from surface-sterilized material bearing quiescent infections. Cultural evidence, together with field observations, indicate that a quiescent infection occurs naturally in the orchard, particularly in a season of moderate to severe blossom infection. Fruit losses during a dry harvest period are ascribed in the main to the activation of quiescent infections during fruit ripening. The histology of the lesion is described. The response of the host to infection is variable, but always includes a more or less extensive collapse and necrosis of the epidermis and of one or more layers of the hypodermis. Periderm formation may be associated with the necrogenic reaction. The results of a spray trial demonstrate that some measure of control of quiescent infection can be achieved by applying eradicant sprays during blossoming, and thus provide circumstantial evidence that a proportion of such infections can be established during this period. The quiescent infection is compared with other host-pathogen combinations in which infection is followed by a period of latency or quiescence, and suggestions are put forward as to the factors governing quiescence. The concept of "latent contamination" is re-evaluated in the light of the evidence presented in the present paper. It is argued that conidia alighting on the fruit surface during the growing period are more likely to give rise to a quiescent infection than to survive to produce an infection of the ripe fruit.