CSIRO PUBLISHING www.publish.csiro.au/journals/app Australasian Plant Pathology, 2006, 35, 619–629 Effects of rootstock and nitrogen fertiliser on postharvest anthracnose development in Hass avocado S. L. Willingham A , K. G. Pegg A , J. M. Anderson A , A. W. Cooke A , J. R. Dean A , F. R. Giblin A and L. M. Coates A,B A Horticulture and Forestry Science, Department of Primary Industries and Fisheries, 80 Meiers Road, Indooroopilly, Qld 4068, Australia. B Corresponding author. Email: lindy.coates@dpi.qld.gov.au Abstract. These rootstock and nitrogen fertiliser studies confirmed that rootstock race can significantly affect the development of postharvest disease and mineral nutrient accumulation in Hass avocado fruit. When Hass (Guatemalan race) was grafted to seedling Velvick (West Indian race) rootstock, the severity and incidence of anthracnose in fruit were significantly reduced by up to 64 and 37%, respectively, compared with seedling Duke 6 (Mexican race) rootstock. Stem-end rot was also influenced by rootstock in some seasons, and significant reductions (up to 87%) in the severity and incidence of stem-end rot were recorded in Hass fruit from Velvick compared with Duke 6 rootstock trees. These improvements in postharvest diseases were associated with significantly lower concentrations of nitrogen and potassium, higher concentrations of calcium and magnesium, lower ratios of nitrogen : calcium and higher ratios of calcium + magnesium : potassium in Hass leaves and fruit from Velvick compared with Duke 6 rootstock trees. Altering the rate of nitrogen fertiliser had minimal impact on postharvest disease development. However, in one season, reducing the rate of nitrogen fertiliser to nil significantly reduced the concentration of nitrogen in the fruit skin, decreased the nitrogen : calcium ratio and significantly reduced the severity and incidence of anthracnose in Hass fruit from both Velvick and Duke 6 rootstock trees. The form of nitrogen fertiliser (ammonium compared with nitrate) applied to the trees did not significantly affect the postharvest disease susceptibility of Hass avocado fruit on either Velvick or Duke 6 rootstock. The Guatemalan race rootstocks, Anderson 8 and Anderson 10, were also found to be superior to the Mexican race rootstock, Parida 1, for reducing anthracnose severity. This again, was associated with a better balance of mineral nutrients (significantly lower nitrogen : calcium and higher calcium + magnesium : potassium ratios) in the fruit. This rootstock effect, however, was only observed in the first season of a 3-year experiment, possibly because of a better balance between vegetative growth and fruit production in Parida 1 in the latter two seasons. Significant positive correlations between anthracnose severity and fruit skin nitrogen : calcium ratios were evident across all experiments. Additional keywords: Colletotrichum gloeosporioides, nutrition. Introduction Postharvest diseases continue to be a major quality issue for the retail marketing of Hass avocado (Persea americana) fruit (Hofman and Ledger 2001). Anthracnose, the most serious postharvest disease of avocado fruit, is caused by the fungus Colletotrichum gloeosporioides (Fitzell et al. 1995). Stem- end rot (SER), the other major postharvest disease of avocado, can be caused by several different fungi including various Dothiorella spp., Lasiodiplodia theobromae and Phomopsis perseae (Fitzell et al. 1995). Colletotrichum gloeosporioides can also cause rots at the stem-end (Fitzell et al. 1995). Postharvest disease symptoms of anthracnose arise from quiescent field infections that do not become apparent until after the avocado fruit is harvested and begins to ripen (Binyamini and Schiffmann-Nadel 1972; Verhoeff 1974). In avocado, quiescence of the anthracnose pathogen is thought to be due to fungitoxic concentrations of preformed antifungal diene compounds present in the skin (Prusky et al. 1982, 1983). Cultivars of avocado naturally more resistant to anthracnose have been shown to have higher concentrations of the diene as well as a slower rate of decline of diene concentration during ripening (Prusky et al. 1988; Ardi et al. 1998). Avocado can be divided into three ecological races: P. americana var. drymifolia (Mexican race), P. americana var. guatemalensis (Guatemalan race) and P. americana var. © Australasian Plant Pathology Society 2006 10.1071/AP06062 0815-3191/06/060619