California Avocado Society 1989 Yearbook  73:71

 

Phytophthora citricola

 

Mike Coffey

Department of Plant Pathology, University of California, Riverside, California.

 

This second Phytophthora problem is becoming more critical. The most urgent needs include the development of knowledge on the choice of a suitable resistant rootstock and evaluation of chemical control methods.

During 1988-89, several different rootstocks with Hass scions were planted at the South Coast Field Station. The rootstocks include Toro Canyon, Martin Grande, Thomas, Barr Duke, Duke 7, and G1033. These have now been inoculated with a highly virulent isolate of P. citricola. Accurate quantitative evaluations will be made of rootstock performance over a ten-year period.

Chemical control evaluation is focused on trunk injection methods with Aliette®.  Earlier experiments demonstrated that Ridomil® used as a ground drench, and either Aliette® or Ridomil® used as a topical trunk spray, were not very effective in preventing the decline of trees infected with P. citricola.

Two new field tests using trunk injections of Aliette® have been under way for two years. This year, a third test was started. Results, though inconclusive, are mildly encouraging in one instance. However, it is too early to assess the efficacy of such methods; a period of five to six years will probably be necessary for field evaluation of fungicides on avocado.

An improvement in our knowledge of how this Phytophthora persists on avocados, and why it suddenly expresses itself as a serious problem, may eventually allow development of more appropriate control measures.