Five isolates of Phytophthora from avocado (Persea americana) were studied and compared with phytophthora citricola. In culture the avocado isolates closely resemble P. citricola. Growth at various temperatures is also similar. Sporangia produced by these isolates vary considerably in shape. “Typical” Phytophthora sporangia of the avocado isolates cannot be distinguished from those of authentic P. citricola isolates, including the type culture; irregular sporangia occur among isolates of both groups but there is a greater irregularity of sporangial form among the former.
The sexual stage of the avocado isolates appears almost identical with that of P. citricola but oogonia and oospores of the avocado isolates are slightly smaller. Electrophoretic protein patterns of the avocado isolates are similar to those of P. citricola. Based on similarities in all the characteristics studied, the avocado isolates are identified as P. citricola. Phytophthora citricola has limited pathogenicity to avocado roots, but can cause stem cankers when wound-inoculated, and produces a rot on unwounded avocado fruit.
Application to control avocado stem canker disease, caused by Phytophthora citricola, was evaluated under greenhouse conditions. Fosetyl-Al was applied to the canker area as paint alone (0.4 g a.i. fosetyl-Al + 1.0 ml water), combined with Tree Seal (0.4 g a.i. fosetyl-Al + 0.5 g Tree Seal + 0.5 ml water), or applied alone followed by Tree Seal on either scraped bark or bark cut in a fish-scale-like pattern (30 cm along the stem). The use of fosetyl-Al as a soil drench (3.2 g a.i. fosetyl-Al per liter) was also evaluated.
The most effective method was either using the fosetyl-Al:Tree Seal:water formulation on heavily scraped areas of the stem or applying fosetyl-Al alone on the bark cut in a fish-scalelike pattern. Applying fosetyl-Al as a soil drench was also effective in controlling stem canker disease, but to a lesser degree than the paint application method. Phosphonate, the anionic metabolite of fosetyl-Al in plants, was quantified in the bark. Leaves, and roots of treated avocado plants by high-performance liquid chromatography following the different application methods of fosetyl-Al. Application of the fosetyl-Al:Tree Seal:water formulation on heavily scraped stem areas resulted in the highest level of phosphonate residue in the canker area and was the most efficacious in controlling the stem canker pathogen.
Phosphonate residue in the plant inhibited infection by P. citricola for about 6 months after its application. There was a strong negative correlation (r = -0.978) between the phosphonate level in the stem bark and the size of the stem canker caused by P. citricola