Effects of potato dextrose broth and gelatin on germination and efficacy of Phoma exigua, a potential biocontrol agent for salal (Gaultheria shallon)

The role of a combination of four gelatin concentration levels (0.0%, 0.2%, 0.5%, and 0.8%) and eight potato dextrose broth (PDB) concentration levels (0.0%, 0.1%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, and 3.0%) on water evaporation, water absorption, conidial germination, and biocontrol efficacy of conidia of Phoma exigua isolate PFC2705 for salal was evaluated. PDB and gelatin reduced water evaporation. After 3 h of evaporation at 23 degrees C and 22% RH, the water content of gelatin-PDB combinations increased compared with PDB alone. The relationships between the amounts of absorbed water and PDB concentration at all gelatin concentration levels were linear (p < 0.001) after 60 min in a 100% RH environment. After 240 min, the mass of absorbed water was 11.8 times the dry mass of the 0.5% gelatin and 1.5% PDB combination, and this relationship did not change significantly over the following 16 h. PDB and gelatin increased conidial germination and might have provided nutrition for conidia of P. exigua PFC2705 at different RH environments. Conidia of PFC2705 did not germinate in sterilized ion-free water, germinated somewhat (4% to 10%) in gelatin suspensions, and germinated well (average 89%) in suspensions of different concentrations of PDB or gelatin-PDB combinations after 24 h at 23 degrees C. The average conidial germination at 24 h was 76% in 100% RH, 12% in 97.5% RH, and 0.0% at 92.5% RH; at 48 h it was 44.4% in 97.5% RH and 20% in 92.5% RH. No conidia germinated when RH was lower than 92.5% over 48 h. Conidia in the gelatin-PDB combinations smeared on slides stored for 4 weeks at 23 degrees C with 22% RH had an average conidial germination rate of 20% after 16 h incubation at 100% RH. Gelatin-PDB combinations with 0.5% gelatin and 1.5% to 2.5% PDB caused significantly larger lesions on detached young salal leaves (20 to 25 days old) than other gelatin-PDB combinations.