Identification and control of Phytophthora collar, crown and root rot of apple cultivars/ rootstocks

Phytophthora cactorum has been shown to be the major causal organism involved in collar, crown and root rot of apples in South Africa.  Identification at molecular level showed that 94% of the Phytophthora isolates from infected apple trees were P. cactorum based on sequence homology to sequences from an international sequence database.  This sequence data of the isolates were used to construct a phylogenetic tree to evaluate the evolutionary relationship amongst the isolates.  A modification of the reverse dot blot hybridisation technique was evaluated as a diagnostic tool for the rapid identification of P. cactorum.  Preliminary results were promising as it showed that P. cactorum could be distinguished from other Phytophthora species.

The excised stem inoculation technique was shown to be a useful technique to determine levels of susceptibility of apple rootstocks to Phytophthora infection.  Of five apple rootstocks (MM106, M25, M793, M7 and M109) tested, MM106 rootstock was the most susceptible towards P. cactorum.  It was also shown that Phytophthora susceptibility is dependent on physiological growth stage of the rootstock.  Resistance in apple seedlings was determined using the zoospore inoculation technique that showed that seedlings younger than five months are more prone to infection than older seedlings. An important observation that was made was the significant decrease in the aggressiveness of isolates in culture.  The P. cactorum toxin, cactorein, was successfully isolated from one of the P. cactorum isolates.  This is significant as the purified toxin can be used to select for Phytophthora resistance in apple rootstocks.

In vitro tests with a mixture of Bacillus spp. showed effective reduction in the growth of Phytophthora cactorum. This result is significant as it shows that Bacillus could possibly be used as a biological control measure against Phytophthora crown rot.