South African Avocado Growers’ Association Yearbook 1995. 18:74-76 Maturity Standards for Fuerte Fruit in the Kwazulu/Natal Midlands C. Kaiser 1 , J. Levin 2 and B.N. Wolstenholme 1 1 Department of Horticultural Science, University of Natal, Private Bag X01, Scottsville, Pietermaritzburg, 3209, RSA 2 Medical Research Council, Private Bag X385, Pretoria, 0001, RSA INTRODUCTION Fruit maturity is a key issue for harvesting and post-harvest handling and avocado fruit are no exception. Physiologically, maturity may be defined as that stage of development at which the fruit, once detached from the tree, will ripen and result in a product desirable for eating (Young & Lee, 1989). Immature fruit are known to be bland and likely to shrivel as they ripen (Bergh et al, 1989), and in extreme cases the fruit may not even soften. Due to the misconception that avocado fruit are very hardy, they are not treated with as much respect post-harvest, as perhaps they should be and it is probably for this reason that less attention has been paid to over maturity of the fruit. Although some cultivars e.g. Hass may be left to hang on the tree for extended periods of time without subsequent deterioration of fruit quality (Kaiser & Wolstenholme, 1994), others may not be suited to delayed harvesting as fruit senescence while on the tree is probably a limiting factor. The fact that Fuerte avocado fruit are more prone to post- harvest physiological disorders than are Hass fruit (Witney et al., 1990) is supporting evidence for this. Consequently, it is imperative that maturity standards, both minimum and maximum be set for these fruit. In the past, the only reliable maturity standard for avocado fruit was found to be total lipid content (Eaks & Sinclair, 1978) and this is still the case today (Kaiser, 1994). Since the total lipids and moisture content are reciprocal and sum to a constant for any one cultivar (Swarts, 1976) the moisture content is still used extensively by the South African industry as a maturity standard. For accuracy however, lipid levels should be determined on a dry mass basis, as the moisture content of the fruit will be determined by prevailing orchard conditions. On a different note however, Zauberman and Schiffmann-Nadel (1972) examined pectinmethylesterase activity (PME), the enzyme responsible for the initiation of the ripening process, in Fuerte fruit at various stages of development and ripening and found that PME activity on the day of harvest decreased with an increase in the stage of fruit development. In younger fruit, PME decreased rapidly while in mature fruit, PME decreased moderately and they suggested from these data that PME may be a possible maturity indicator. Apparently however, no further work has been done in this regard. Consequently, the present study was undertaken to determine whether PME is a suitable maturity marker for Fuerte fruit.