Inoculum and Infection Dynamics of Polystigma amygdalinum in Almond Orchards in Spain

Red leaf blotch (RLB) disease of almond, caused by Polystigma amygdalinum, is an important foliar disease in most production regions of the Mediterranean basin and the Middle East because severe infections may cause a premature defoliation of the tree. Some key aspects on the epidemiology of P. amygdalinum were studied in multiyear trials in two almond-growing regions in Spain, which included the seasonal development of perithecia and production and germination of ascospores along with the disease incubation and plant infectivity periods. Our results showed that primary inoculum was available in extended periods (January to August). Significant differences in ascospore amounts among regions, higher in the southern Andalusia and lower in the northern Catalonia, and years of study were detected. The factors geographical location, sampling period, and evaluation year were found significant on the development of P. amygdalinum perithecia. Variable ascospore germination rates were observed from April to July: >15% but rarely exceeding 30%. The RLB infectivity period in Catalonia extended from March to mid-June, whereas in Andalusia it was from March to May. The incubation period was mainly in a range of 5 to 10 weeks in Catalonia. The environmental conditions of October to January influence the available ascospore amounts in the next season. RLB infection occurs in spring to summer when mean temperatures are in the range 10 to 20°C. These results represent the first step in developing a prediction model of the disease that might serve as a tool for the control of RLB.