Seasonal fluctuations in the extent of colonization of avocado plants by the stem canker pathogen Phytophthora citricola

At monthly intervals, plants and stem cuttings of avocado (Persea americana Miller) 'Hess' grafted on 'Barr Duke' rootstock and 'Topa Topa' growing in a lathhouse were wounded and inoculated with the stem canker pathogen, Phytophthora citricola Sawada. The seasonal changes (measured monthly) in the extent of colonization of the avocado plants by P. citricola followed a periodic pattern, with two peaks of colonization during an annual growth cycle. Concentration of free amino acids and total soluble carbohydrates in the plant tissue followed a periodic pattern with two peaks similar to that of canker growth. Months were significantly different for canker size, free amino acids, and total soluble carbohydrates of the bark tissue. The extent of colonization was highest during May-June, after the first vegetative flush, and during November-December, after the second vegetative flush. Total free amino acids of the bark tissue was highly correlated with canker size (r = 0.89). Although the total soluble carbohydrate of the bark tissue was also elevated during the periods of canker development, it showed lower positive correlation (r = 0.45) with canker size. Plants were relatively resistant to colonization through March-April, during the first vegetative flush, and through August-September, during the second vegetative flush. Cankers formed on stem cuttings were generally larger than those of intact plants