NATURAL AND INDUCED RESISTANCE OF TABLE GRAPES TO BUNCH ROTS

Abstract

Differential susceptibility of grapes to bunch rots may arise from either physiological or anatomical sources. Although all cultivars show an increase in susceptibility with growth and development, varietal differences have been observed, most noticeably in mature fruit. The causes of these differences have been the subject of study from an anatomical and physiological standpoint, and a number of factors have been found to correlate significantly with apparent resistance of the fruit to disease. Anatomical examination of the skin of different cultivars has demonstrated a correlation between the resistance of grape berries to artificial inoculation with Rhizopus stolonifer and the thickness of the skin, as derived from the number of cell layers and cell density in the epidermis and hypodermis. These attributes were manipulated by applying growth regulators, thus affecting the susceptibility to fungal decay. The angle formed between the fruit and the pedicel was shown to be a morphological factor correlated with resistance. A sharp angle, which enables spore accumulation and a suitable microclimate, was conducive to fungal development, whereas an obtuse angle did not allow such conditions to occur, and the berry remained healthy following artificial inoculation by immersion in a spore suspension. Varietal and temporal differences in tannin content correlated well with disease resistance, as did similar differences in the potential for phytoalexin production. It appears that all these factors may contribute to the resistance of grape berries to fungal invasion and development, and that at each stage of development, the relative importance of each factor changes.