The effect of leaf wetness duration and temperature on the infection of young grapefruit leaves by conidia of Phomopsis citri was investigated. The optimum temperature for infection and symptom development was 24-27°C range. Relative short wetness durations (4-8 h) resulted in some infection, but severity was low. More than 12 h of leaf wetness are required even at optimal temperatures for high levels of infection. Maximal levels are attained only at 24 h of leaf wetness. Field studies utilizing potted trap plants were conducted to determine the relationship between rainfall and temperature in the production of melanose symptoms.
Products which induce systemic resistance in plants (SIR compounds) were evaluated in greenhouse tests for melanose control. Most of these products were not highly effective for control of this disease, but when applied prior to inoculation, many reduced disease severity. ReZist, Serenade, and several other products are effective in this regard (Plant Dis. 87:69-74). We have determined the baseline sensitivity of Diaporthe citri to azoxystrobin and pyraclostrobin in vitro. This information should be useful in assessing the development of resistance to these fungicides (Plant Dis. 89:1186-1194).
Inoculum production of Phomopsis citri on detached twigs under various conditions was investigated. Soaking twigs with melanose symptoms every other day maximizes production of pycnidia. The optimum temperature for pycniduim production was 28°C. Pycnidial production on twigs was proportional to melanose severity on the twigs and no pycndia were produced on asymptomatic twigs. Pycnidial production was greatest on twigs 3-5 mm in diameter and less on thinner or thicker twigs. Melanose-affected twigs were cut each month for two years and placed in mesh bags and hung in the tree canopy. Maximum pycnidial production was on twigs placed in the tree in the winter that sporulated most profusely from April to July.
We have also found that healthy twigs that have died can later be colonized by D. citri and produce large amounts of inoculum. Spore tendrils that ooze from pycnidia often dry and conidia within them can survive for extended periods. Conidia are then dispersed with heavy rains. Once dispersed, conidia only survive for short periods of time.
