PATHOLOGICAL STUDIES ON CROWN CANKER AND ROOT-ROT OF APPLE INCITED BY Phytophthora cactorum

Abstract

Natural infection with crown canker (Cc) and root rot (Rr) was observed on apple trees and seedlings grown in different locations. Isolation trials from collected diseased samples yielded many fungal isolates. Phytophthora cactorum (Pc) isolates were found to be the most dominant ones. Pathogenicity tests of eight Pc isolates on MM I 06 and Malus rootstocks revealed that they were all pathogenic and both isolates No. I and No. 3 were the most virulent, therefore they were used in the following experiments. The tested two Pc isolates caused infection to both apple and pear seedlings. Gradual increase in the inoculum level of the two tested isolates caused also gradual increase in Cc and Rr severity. Severity of Cc and Rr gradually decreased by the increase of rootstock age. The highest Cc and Rr severities were recorded on MM 106 rootstocks grown in clay soil, while the lowest infection was observed on rootstocks grown in soil consisted of 4 sand and 1 clay. Regarding natural soil, the highest infection occurred on rootstocks grown in loam soil. Meanwhile, the lowest infection was recorded on rootstocks grown in clay loam soil. Severities of Cc and Rr were gradually decreased by increasing irrigation intervals. All the used eight Pc isolates were able to grow on the tested eight solid media. The best media were com meal agar and potato agar, while both carrot agar and tomato agar media were the least suitable. Maximum mycelial weight was obtained when com meal medium was used followed by oat-meal, potato and carrot media. All Pc isolates failed to form any sporangium on oat meal agar medium and maximum production was achieved by isolates No. 1, No. 3 and No. 4 on onion agar medium. Moreover, all (Pc) isolates failed to form any oospore on both oat meal and com meal agar media, with the exception of isolate No. 5 when grown on com meal agar medium. All Pc isolates can grow at 10-30°C, while four of these were able to grow at 35°C on oat meal agar medium. Enzymatic activities of the tested Pc isolates revealed that PG enzyme activity was increased within 15 minutes. This increase was further recorded after 30 minutes with the exception of isolates No. I and No. 3, which recorded their maximum increase after 15 minutes. Moreover, great fluctuation was recorded for PME activity by the tested Pc isolates. Furthermore, in most cases Cx activity differed among the tested isolates, with the exception of isolate No. 8 which no Cx.