One of the goals of our research program is development and utilization of a weather-based model for prediction of Alternaria brown spot. We have completed work on the effects of environmental factors on the production and release of conidia and on the seasonal patterns of disease development in the field (Phytopathology 88: 1218-1223). In addition, we have determined the effects of leaf wetness and temperature on the infection of young leaves in the laboratory by different isolates of Alternaria sp. (Plant Dis. 83: 429-433).
More recently, we have collected disease severity data in the field on a daily basis. We related infection to rainfall, leaf wetness, temperature, and to the number of conidia trapped. From these data we developed a model that has been useful in timing of fungicide applications. The model is called the Alter-Rater and predicts disease severity based on rainfall, leaf wetness, and temperatures for each day. Points are assigned for each day and totaled and sprays are applied after accumulation of a certain number of points (Plant Dis. 84: 638-643). We evaluated thresholds for Alter-Rater in several groves in Florida and found that the Alter-Rater usually reduced the number of sprays required or improved fruit quality (Plant Dis. 87:1089-1093). We have evaluated the Alter-Rater and the relationship of disease severity and yield in Brazil.
We have evaluated the in vitro effects of azoxystrobin and pyraclostrobin on A. alternata and established baseline sensitivities to these fungicides. These values should be useful for documentation of the occurrence of resistance (Plant Dis. 89:1186-1194). We have also evaluated the effects of leaf age, humidity and fungicide application on sporulation of A. alternata. The relative tolerance of the different cultivars is being determined and it appears that the leaves of the more susceptible cultivars are sensitive until they are nearly mature whereas those leaves less susceptible cultivars are sensitive only when they are young.
Population genetics, taxonomy, and ecology
We are collaborating with Tobin Peever (Wastington State University) and Kazu Akimitsu (Kagawa University, Japan) to investigate the population biology, ecology, and evolution of pathogenicity of Alternaria on citrus. Our work is directed towards developing an understanding of the relationship between the tangerine pathotype, the rough lemon pathotype, saprophytic, and black rot isolates of Alternaria.
We have collected Alternaria isolates from Minneola tangelo and some other hosts from Florida, Turkey, Israel, South Africa, Colombia, and Australia. We have evaluated the pathogenicity and the RAPD patterns of these isolates. We have sequenced segments of the endopolygalacturonase gene to determine the relationship of these isolates. Isolates group into three clades with 1 from the US and Colombia, group 2 from the US, Australia, Israel, and S. Africa and group 3 from Turkey, Israel, S. Africa, and Australia (Phytopathology 92:794-802). We have confirmed the presence of Alternaria brown spot in Argentina and Brazil (Plant Dis. 87: 750), in Peru (Plant Dis. 90: 686) and Iran (New Dis. Rep. 12: 2005-98) and it has been documented in Spain and Italy as well.
We are also interested in the taxonomic classification of citrus isolates of Alternaria. Simmons (Mycotaxon 70: 263-323) has described 10 species from our collection of isolates on the basis of the morphology of conidial chains. We have used RAPD data and sequence information to make comparisons among citrus isolates and to compare citrus isolates with isolates of known identity from other hosts. We concluded that all citrus isolates with the exception of A. limicola from Mexican lime in western Mexico form a single group and decided to include all in the species A. alternata. (Mycologia 96:119-134). We have compared the potential of isolates to cause black rot of fruit and found that all citrus isolates as well as some from other hosts can cause black rot. Sequence information failed to identify clades unique for isolates causing black rot. All isolates were included in A. alternata and use of the designation of A. citri as the causal agent for black rot appears unwarranted (Phytopathology 95:512-518).
Akimitsu and his colleagues in Japan have cloned two genes controlling the synthesis of the AK-toxin of A. alternata on Japanese pear. Using heterologous probes, homologs were detected in isolates from Minneola tangelo but not in isolates from rough lemon or in saprophytic isolates (Phytopathology 90: 762-768). Toxin genes are located in a cassette on one very short choromosome in all isolates. We have identified and characterized an isolate from rough lemon that is pathogenic to rough lemon and tangerine. The isolate has genes for production of both toxins (Phytopathology 95:241-247).
