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ISHS Acta Horticulturae 264: II International Protea Research Symposium

CANKER AND DIE-BACK OF CUT-FLOWER PROTEAS CAUSED BY BOTRYOSPHAERIA DOTHIDEA: EPIDEMIOLOGY AND CONTROL.

Authors:   S.L. Von Broembsen, J.A. Van der Merwe
Abstract:
Stem canker and die-back caused by Botryosphaeria dothidea result in considerable losses in the prdouction of cut-flower proteas in South Africa. Field and laboratory studies were carried out to investigate conditions favouring the disease and methods of control. The seasonal pattern of B. dothidea sporulation in a naturally infected planting of Protea grandiceps was determined by weekly trapping and counting of conidia for one year. Climatological data such as temperature, rainfall, and relative humidity were recorded throughout the trial. Sporulation occurred from spring to late summer following rain. Sporulation was negligible during winter months despite abundant rainfall. The most significant factors affecting sporulation were average daily temperatures above 20 C and rainfall. The optimum temperature for radial growth of B. dothidea on malt agar plates was 25 C. Conidial production on malt agar and germination in distilled water were greatest at 20–25 C. Infection studies using potted seedlings of P. grandiceps and P. cynaroides in controlled environment cabinettes showed that wounding was necessary for infection and that infection was greatest at 25–30 C.

The effects of three fungicides (benomyl, captab, and mancozeb) and six fungicidal wound sealants on mycelial growth, conidial production and conidial germination by B. dothidea were examined in the laboratory. Benomyl added to media at 1000 ppm completely inhibited all three processes, and Hostaseal gave the best inhibition of the wound sealants. However, benomyl applied as a spray following pruning gave better field control of the infection of pruning wounds than Hostaseal. In a field trial combining several different control strategies, monthly spray applications of the pesticides, chlorophyriphos and dimethoate, in combination with benomyl and captab gave better control than the two fungicides alone. This supports field observations that cankers are often associated with insect wounds. However, this trial also showed that the single most effective control measure was regular removal and destruction of all dead and dying plant parts. Control of Botryosphaeria canker can thus be brought about by preventing unnecessary wounding of plants, by treating harvesting and pruning wounds with a fungicidal spray of benomyl, and by regular sanitation of plantings.

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