Atypical late blight symptoms following first recorded infections by Phytophthora infestans genotype EU_39_A1 in UK vine tomatoes

Early in 2018, Phytophthora late blight was recorded on young plants in at least five large UK vine-tomato nurseries. Infection (as high as 20%) appeared limited to some batches of plants. Initially, affected plants showed some foliar and girdling stem lesions with a high rate of mortality. These were rogued and no secondary infection appeared to occur. Some infected plants (<1%) survived. These had no foliar or fruit lesions. They were commercial grafts with two heads per rootstock generated by pinching the scion just above the cotyledons. Lesions started at the graft, generally affecting a single head per plant (Fig. 1), and extended up stems throughout the season at an average rate of 40-60 mm per week (Figs. 2-3). Tan-brown lesions, superficially resembling Botrytis stem rot, affected the outer stem layers (Fig. 4), not penetrating far into the vascular tissues and allowing shoot growth. Stem diameter was reduced compared to healthy vines, and fruit yield was reduced by about 25%