Control of Diplodia pinea and D. scrobiculata in Pinus halepensis by 5-chloro-salicylic acid

Abstract

Diplodia pinea (syn. Sphaeropsis sapinea) and D. scrobiculata are destructive pathogens of conifer species in many parts of the world. The sensitivity of these fungi to externally applied 5-chloro-salicylic acid on Pinus halepensis was studied. Trees treated with 2 mM 5-chloro-salicylic acid were more resistant to the fungi than untreated trees. After 15 days of treatment shoot dieback affected 30% of trees inoculated with D. pinea, compared to 60% of untreated trees. D. scrobiculata caused shoot dieback in 30% of untreated trees but only in 20% of trees pretreated with 5-chloro-salicylic acid. The controls never developed tip blight. The direct effect of 5-chloro-salicylic acid on the mycelial growth of D. pinea and D. scrobiculata was tested in vitro using PDA amended with 5-chloro-salicylic acid at five concentrations (0.2, 1, 2, 2.5, and 3.0 mM). The radial growth of colonies was measured after 48 and 72 h of incubation at 24°C. After 48 h, 5-chloro-salicylic acid significantly inhibited mycelial growth of D. pinea at 3mM, although there was no longer any significant difference in growth rates after 72 h of incubation. D. scrobiculata was slightly more sensitive to 5-chloro-salicylic acid than D. pinea. After 48 h, significant differences were observed in the mean colony diameter of D. scrobiculata when directly exposed to 5-chloro-salicylic acid at dilutions from 0 to 3 mM. After 72 h, however, mycelial growth was reduced significantly only at the highest concentrations (2; 2.5 and 3 mM) (P-value <0.05).