2.4.7
EPIDEMIOLOGY OF FUSARIUM STEM AND FRUIT ROT OF GREENHOUSE-GROWN SWEET PEPPER

RJ COPEMAN and MIREJ SMUDJA

Plant Science Department, University of British Columbia, Vancouver, BC, Canada V6T 1Z4

Background and objectives
Fusarium stem and fruit rot, caused by Nectria haematococca, has been responsible for losses ranging between 1 and 2 ;kg/m2 ($30,000 to $60,000/ha). Because it has generally been considered a minor disease associated with poor climate management, no special control measures have been recommended. Details of the disease cycle have not been published. The objective of this work was to establish how the pathogen survives and spreads and to determine the conditions favouring dissemination and infection.

Materials and methods
Burkard Continuous Recording Air Samplers were placed in the canopy of 'problem' commercial pepper greenhouses to determine the form of the primary and secondary inoculum and to monitor spore numbers during the growing season. The effectiveness of the end-of-season cleanup procedures in 'problem' greenhouses was similarly evaluated. The effect of temperature and relative humidity on ascospore discharge from diseased pepper fruits and ascospore germination were determined in vitro under controlled environments [1,2].

Results and conclusions
Spore monitoring in commercial pepper crops during three different growing seasons has established that ascospores of N. ;haematococca are both the primary and secondary inoculum. Ascospores are discharged only during the night and the duration of discharge varies with photoperiod during the growing season. No ascospores could be detected in 'problem' greenhouses upon completion of the end-of-season cleanup procedures, indicating that detectable spore inoculum was not being carried over from one season to the next in the commercial greenhouses. With the introduction of certain lots of seedlings into 'clean' greenhouses the ascospore levels trapped were higher than those observed in the old crop. No symptoms were observed on the new seedling plants but perithecia of N. ;haematococca were observed growing on the surface of the rockwool cubes in which the seedlings were growing. Spores trapped from these perithecia were pathogenic to pepper seedlings and caused typical fruit rot. Rockwool cubes infested during propagation are a source of primary inoculum in 'clean', commercial greenhouses and are implicated as the probable source of the background level of ascospores recovered throughout the growing season in greenhouses not having a disease problem owing to aggressive climate management.

Under controlled conditions of temperature and relative humidity ascospores were discharged from diseased fruit tissue at temperatures ranging from 15 to 30C and relative humidities from 51% to 100%. Maximum discharge occurred at 15C and 100% relative humidity, which is consistent with spore monitoring data from commercial greenhouses. Ascospore discharge from the rockwool cubes is independent of ambient relative humidity. Ascospores discharged onto glass surfaces maintained at a relative humidity of 55-60% (typical relative humidity observed during cleanup) survived from 6 to 18 ;days depending upon temperature. Ascospores of N. ;haematococca are quite hardy and can remain viable on glass surfaces for at least 14 ;days under typical greenhouse conditions. Only 10% germination was observed in ascospores exposed to a constant 95% relative humidity for 3 days at 20C. Greater than 50% ascospore germination occurred only under prolonged periods of >98% relative humidity. Climate management as a control strategy appears to exert its effect by preventing spore germination rather than preventing spore dispersal.

References
1. Semeniuk G, 1993. Plant Disease 77, 37-42.
2. Arauz LF, Sutton TB, 1989. Phytopathology 79, 667-674.