Seasonal Variation in Susceptibility of Citrus Rootstocks to Phytophthora

Abstract

Phytophthora parasitica and P. citrophthora are routinely recovered from diseased citrus groves in Arizona. Stem sections were collected monthly from Citrus macrophylla, rough lemon, .sour orange, Cleopatra mandarin, Troyer citrange and Citrus volkameriana. Stem pieces were wounded, inoculated with mycelium of P. parasitica or P. citrophthora, then incubated for 7 days at 21° C in moist chambers. For all tested rootstocks, the smallest cankers were produced on tissue collected in December, January and February, the winter dormancy period for citrus in Arizona. The period of higher susceptibility ranged between March through November, depending on the rootstock tested. Apparently, these six citrus rootstocks possess seasonal differences in their susceptibility to P. parasitica and P. citrophthora.