Optimizing a Seedling Screening Method for Anthracnose Resistance in Watermelon

Abstract

Among three races of Colletotrichum orbiculare, which cause anthracnose of cucurbits, screening for race 2 resistance was studied under greenhouse conditions at various inoculum concentrations, and plants were rated on different days postinoculation (DPI). The objectives of this study were optimizing inoculum concentration and phenotyping DPI for seedling resistance. Five inoculum concentrations were compared (2.5 × 104, 5 × 104, 1 × 105, 2.5 × 105, and 5 × 105 conidial spores/ml). Four watermelon genotypes (‘Black Diamond’, ‘Charleston Gray 133’, PI 543210, and PI 189225) and two cucumber genotypes (‘Marketer’ and ‘H19 Little Leaf’) were evaluated. Disease was recorded on the percentage of cotyledon area lesion, severity of hypocotyl lesion (SHL), severity of petiole lesion (SPL), and percentage of leaf area lesion (PLL), as well as a disease index (INDX) from 5 to 14 DPI. There was a significant difference among genotypes and inoculum concentrations. The resistant PI 189225 was significantly different (P < 0.05) from the highly susceptible PI 543210. Inoculum 1 × 105 spores/ml was at par with 5 × 105 and 2.5 × 105 but significantly different from 5 × 104 and 2.5 × 104 for AUDPS PLL, AUDPS INDX, AUDPS SPL, and AUDPS SHL. Inoculum at 1 × 105 spores/ml was optimal to differentiate germplasm. A genotype plus genotype-by-environment biplot showed that PLL was a representative rating. A single PLL rating on 9 DPI would optimize resources for screening a large set of germplasm for anthracnose resistance in a watermelon breeding program.