Effects of Temperature on Events in the Infection Cycle of Two Clonal Lineages of Phytophthora infestans Causing Late Blight on Tomato and Potato in Brazil.

In Brazil, US-1 and BR-1 clonal lineages of Phytophthora infestans are widely distributed throughout the major growing areas of tomato and potato, respectively. Quantitative information regarding the effects of temperature (10, 15, 22, and 27°C) on direct and indirect sporangia germination, incubation period (IP), latent period (LP), lesion area (LA), and sporulation (SP), as well as combined temperature and leaf wetness duration periods (6, 12, 18, and 24 h) on the number of lesions (NL) was obtained under controlled conditions on either detached leaflets or whole plants. The percentage of indirect germination was higher for BR-1 isolates than for US-1. The percentage of direct germination was higher for US-1 than for BR-1. The shortest IP and LP were recorded at 22°C for both lineages: 69.3 h and 93.3 h for US-1 isolates on detached tomato leaflets, and 44.0 h and 68 h for BR-1 isolates on detached potato leaflets, respectively. US-1 isolates did not sporulate at 10°C, and BR-1 isolates did not sporulate at 27°C. Isolates of both lineages induced the largest LA at 22°C. The NL was highest at 15°C for US-1 isolates on whole tomato plants, and at 10°C for BR-1 isolates on whole potato plants. The differential effects of temperature on US-1 and BR-1 suggest that current decision support systems initially developed for controlling US-1 in Brazil may now be inaccurate for controlling BR-1, and management strategies should be properly validated before being used.