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Title: ENVIRONMENTAL FACTORS AFFECTING PRODUCTION, RELEASE, AND FIELD POPULATIONS OF CONIDIA OF ALTERNARIA ALTERNATA, THE CAUSE OF BROWN SPOT OF CITRUS

Author
item TIMMER, L. - UNIV. OF FLA., IFAS
item SOLEL, Z. - VOLCANI CENTER, ISRAEL
item Gottwald, Timothy
item IBANEZ, A. - UNIV. OF FLA., IFAS
item ZITKO, S. - UNIV. OF FLA., IFAS

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/3/1998
Publication Date: N/A
Citation: N/A

Interpretive Summary: Alternaria brown spot of tangerines can cause severe crop reductions where it occurs. The disease appears to be on the increase in Florida tangerines. To attempt to control the disease those, factors, which cause its increase and dissemination, need to be studied. These are primarily spore production and spore release. We set up a series of field and laboratory experiments to determine the environmental triggers for spore production and release. Spore production was greatest when foliage was maintained in a constant moist condition for 24 hours. We also found that sudden shifts in relative humidity and brief rainfalls were the most stimulatory to spore release. By determining the environmental factors, which stimulate inoculum production and pathogen, spread, we hope to design horticultural methods that can be used to minimize the stimulatory effect of these environmental conditions and thus minimize disease. For instance tighter control of irrigation practices. In addition we hope to use this information to more effectively time and minimize chemical applications for control.

Technical Abstract: Alternaria brown spot causes lesions on immature leaves and fruit of many tangerines and their hybrids reducing yield and marketability of the fruit. Conidial production was greatest on mature leaves moistened and maintained at high RH for 24 h, whereas leaves which had been soaked or maintained at moderate RH produced few conidia. Conidial release from filter paper cultures and infected leaves was studied in a computer-contro nvironmental chamber. Release was triggered from both substrates by sudden drops in RH or by simulated rainfall events. Vibration and exposure to red/infrared irradiation stimulated release of only low numbers of conidia. In field studies from 1994-96, conidia were captured throughout the year in a Burkard-type spore trap with periodic large peaks. The number of conidia captured was not closely related to rainfall events, proportion of hours with leaf wetness or wind speed. Likewise, infection of trap plants placed in the field weekly during 1995-95 was not closely related to conidial numbers, rainfall, or leaf wetness. Infection appears to occur throughout the year whenever susceptible tissue and moisture are available.