RELATIONSHIP OF PHYTOPHTHORA FRUIT ROT TO FRUIT MATURATION AND CUTICLE THICKNESS OF NEW MEXICAN-TYPE PEPPERS

Experiments were conducted to determine the relationship between pepper fruit susceptibility to Phytophthora fruit rot and fruit maturity. Pepper fruits were harvested weekly throughout the 1991 growing season and were dipped in a suspension of Phytophthora capsici zoospores (5 X 10(3) zoospores / ml). The susceptibility of nonwounded fruit to infection decreased with increased ripening. Lesion elongation in nonwounded fruit decreased from 14.1 mm/day in green fruit to 10.7 mm/day in nondehydrated red fruit. Nonwounded fruit (immature green, mature green, and red) inoculated with 5 X 10(3) zoospores in a 100-mul drop also exhibited less infection as the fruit ripened. Rates of lesion elongation were 9.6 mm/day, 5.9 mm/day, or 2.7 mm/day in nonwounded immature green, mature green, or red fruit, respectively. In contrast, lesion growth rate in wounded fruit was 14.0 mm/day for all maturation stages. Fungal growth was greater in red fruit extracts than in green fruit extracts, which suggested that inhibitory mechanisms, such as fungal chemical inhibitors, were not present in red fruit. Fungal growth was directly related to the sugar content of the fruit. Cuticle thickness, however, increased from 12 mum in immature green fruit to 24 mum in red fruit. Peroxidase activity and isoforms increased with ripening. Nitrocellulose tissue blotting indicated increased peroxidase activity in the cuticular region as the fruit matured, which suggested increased polymerization of ligninlike compounds and an increased barrier to infection. Cuticle thickness of nonwounded mature green and red pepper fruit appears to be a factor in resistance to P. capsici.