We have investigated the effect of temperature and leaf wetness on infection of rough lemon leaves. Seedlings in small pots are cut back to force new growth and then inoculated with conidia of Sphaceloma fawcettii. The optimum temperature for infection and symptom development is 24-27oC. Some infection occurs with as little as 2-4 h of leaf wetness duration at optimum temperature. However, disease incidence and severity increased with up to 12 h of leaf wetness (Plant Dis. 87:1102-1106).
We are cooperating with the laboratory of J.W. Hyun at the Jeju Agricultural Experiment Station, Jeju , Korea, and K.R. Chung (CREC, Lake Alfred) in a project to better understand pathogenesis of Elsinoe spp. on citrus and to determine the relationships among species and pathotypes of the fungus. We have developed a transformation system for Elsinoe and induced avirulent mutants and to seek the genes responsible for pathogenesis. We will also determine the role of toxins and hormones in pathogenesis.
A collection of isolates from many areas of the world has been assembled in Korea and those isolates are being inoculated on a range of citrus species in an attempt to identify new pathotypes of this fungus. These isolates are also being compared using RAPD analysis to help determine their phylogenetic relationship. We have developed primers that are specific to E. fawcettii and to E. australis and to the natsudaidai pathotype of E. australis. Those primers are useful for identifying cultures of these species using mycelium as a source material and also for detection and differentiation of species directly from leaf and fruit lesions.
We investigated the activity of compounds producing systemic induced resistance (SIR) in plants for activity against scab. Greenhouse seedlings were treated with the materials being tested at various rates and treatment schedules and then are inoculated with conidia of Sphaceloma. Compounds such as Oxycom, Messenger, and ReZist reduced scab severity when applied prior to inoculation. However, some products did not perform consistently (Plant Dis. 87:69-74). We have also determined the baseline sensitivity of E. fawcettii to pyraclostrobin, and fenbuconazole. The baseline values should be useful in evaluating the occurrence of fungicide resistance (Plant Dis. 89:1186-1194).
