Characterisation, Epidemiology and Control of Scab on Protea Spp. Caused by Elsinoe Spp. (Anamorph Sphaceloma Protearum).

Abstract

Protea scab, a commercially important disease of Protea spp., is discernable by raised, circular to oval, red – black leaf and stem lesions, which distort young plant growth. The causal agent was determined to be Sphaceloma protearum (Teleomorph Elsinoë spp.). Erumpent conidophores and acervuli in diseased tissue were characteristic of the Loculoascomycete Elsinoë. The teleomorph state was rarely observed. Ascospores were hyaline, ellipsoid, 4 transversely septate, 0-1 vertical septa, 11-13(-16) x 4-5 μm. Elliptical, aseptate, hyaline conidia were obtained in vitro in Fries’s medium. S. protearum is an extremely slow growing fungus, 2-5 mm in 14 days on malt extract agar (MEA) at 25 ºC and 32-90 % relative humidity (optimum growth conditions). Maximum cardinal temperature was below 30 ºC. Colonies were irregular, raised and convoluted, with smooth colony margin, red-brown in colour and released a red pigment into the agar. Chlorothalonil, at 1000 ppm, was added to MEA to control growth of contaminants. It had no effect on S. protearum growth. Protea compacta x susannae cv Pink Ice developed typical disease symptoms 3 to 4 weeks after inoculation by insertion of a mycelium plug into stem tissue, from which S. protearum was re-isolated. Additional hosts not previously recorded were: P. grandiceps x eximia cv Rosie, P. compacta x burchellii cv Brenda, P. longifolia cv Satin Pink, P. compacta x obtusifolia cv Red Baron, P. repens cv Guerna, P. laurifolia cv Regal Mink, P. neriifolia cv Silvertips and P. F2 hybrid cv Niobe. The pathogen over-wintered in old scab lesions as ascospores and / or resting mycelium. Infection was by wind and water dispersed propagules. Plants under stress, such as lack of water, were particularly susceptible to infection. In vitro fungicide trials of 3 isolates revealed that EC50 for mycelial growth was below the 100 % inhibition level of the recommended field concentration (IRC) for captan and prochloraz (all isolates), mancozeb (isolate J24) and azoxystrobin (isolate D01). Tolylfluanid offered no protection against S. protearum. In the field good control of protea scab on P. cv Pink Ice was obtained with azoxystrobin (250 mg/l) and prochloraz (500 mg/l), with AUDPC of 1.39 and 1.43 respectively. This was followed by the positive control (1.84), mancozeb (1.96) and lastly [captan + benlate] (2.06). Plants should be sprayed when actively growing, and during hot, moist weather conditions (20-27 ºC, 45-90 % humidity). Cultural methods are also discussed.