Septoria leaf spot of black currants

Septoria leaf spot – Mycosphaerella ribis
The fungus attacks leaves, shoots and berries causing small dark spots which develop a pale brown centre with a darker brown margin, making them easily distinguishable from anthracnose leaf spot. Black pin-head sized fruiting bodies (pycnidia) become embedded in the spot centres, mostly on the leaf underside.
Importance
Important leaf spot disease of blackcurrants in Lithuania, Finland, Russia, New Zealand, Poland, Latvia, Bulgaria, Tasmania, but of minor significance in UK. If not controlled the disease causes premature leaf drop, reduces shoot growth and crop yield. The fungus is also reported to produce a toxin that affects shoot growth. Studies by Lebedeva and Ivanteeva showed that blackcurrant bushes infected by Septoria ribis had reduced photosynthesis and increased respiration and transpiration. Chlorophyll, carotene and sugar concentration of infected bushes were also reduced.
Epidemiology
The fungus overwinters on leaves on the ground as sclerotia and sexual fruiting bodies (perithecia). Survival of these is influenced by weather conditions during the winter. Wet conditions favour leaf decomposition. Ascospores mature in April / May and are released from the perithecia to infect leaves in spring, usually coinciding with bud burst, which in eastern Europe is generally around May, western Europe in April and in New Zealand late August / September. In June leaf spots appear which eventually bear the pycnidia from which conidia are produced and spread by rain splash to infect other leaves during the summer. The incubation period from infection to symptoms varies from 19-26 days in June. The optimum conditions for fungal development are 20-25 C with reductions in growth below 10 C and 30 C. Conidial production occurs between 15-20 C. Septoria ribis exists in various physiological races that vary in pathogenicity.
Disease control
Many studies have been conducted on control of Septoria leaf spot. Lebedeva and Ivanteeva demonstrated that certain fertilisers, in particular urea and 5 and 10% potassium chloride markedly decreased infection of blackcurrant by Septoria. Most other studies have focused on fungicide control. Spray timing was important to achieve control with early control of primary infections from ascospores most critical. Fungicides applied post harvest to control late infection had no effect on yield the following season. Effective fungicides identified included tolylfluanid and penconazole. Thiram, triforine, folpet, mancozeb and prochloraz were also shown to give a high level of disease control, increasing crop yield and delaying post harvest defoliation.