Screening Potential Citrus Rootstocks for Phytophthora nicotianae Tolerance

Seeds from four citrus rootstocks including sour orange, Bitters-C22 citrandarin, Sarawak pummelo x Rio Red grapefruit, and Sarawak pummelo x Bower mandarin were exposed to high inoculum levels of Phytophthora nicotianae to screen for tolerance. Inoculation of pregerminated seeds (PGIS) and non-PGIS was carried out. The average P. nicotianae propagule counts from the soil samples where these seedlings were raised ranged from 424 to 1361 colony forming units/cm(3). The proportion of live to dead plants was recorded at 11 months postinoculation, which showed that Sarawak x Bower performed significantly better than other rootstocks. Evaluation of the rootstocks 18 months postinoculation resulted in only one surviving sour orange plant, which suggests potential rootstock resistance.