Alternaria diseases of citrus - Novel pathosystems

Citrus is affected by four diseases caused by Alternaria spp. Brown spot of tangerines, leaf spot of rough lemon, postharvest black rot of fruit occur widely in citrus areas of the world and are caused by different pathotypes of A. alternata. Mancha foliar occurs only on Mexican lime in western Mexico and is caused by A. limicola. Tangerine and rough lemon pathotypes produce host-specific toxins that affect membranes and respiration, respectively. Black rot is always associated with wounds and is caused by most citrus-associated isolates of A. alternata that produce endopolygalacturonase. Alternaria brown spot is a serious disease of susceptible tangerines and their hybrids in semi-arid Mediterranean climates as well as in more humid areas. Conidia, produced on lesions on mature and senescent leaves and stems under humid conditions, are dispersed by wind, and infect all juvenile tissues of susceptible cultivars when temperature and leaf wetness conditions are favorable. Commercially acceptable cultivars resistant to brown spot are being developed. Disease severity can be reduced by planting disease-free nursery stock on wider spacings, pruning tree skirts, and reducing irrigation and nitrogen fertilization. However, fungicides such as dithiocarbamates, triazoles, strobilurins, iprodione, or copper fungicides are used in most areas for disease control. A disease-forecasting model, the Alter-Rater, has been developed in Florida to assist in timing fungicide sprays

Temperature, leaf wetness, and isolate effects on infection of Minneola tangelo leaves by Alternaria sp.

Alternaria brown spot causes necrotic lesions on immature leaves, twigs, and fruit of tangerines and their hybrids, reducing yield and fruit quality. The effect of temperature, leaf wetness, and isolate was evaluated in an in vitro system using immature detached leaves of Minneola tangelo. Infection was greatest at 27°C, decreased gradually as the temperature declined to 24, 20, and 17°C, and dropped sharply at 32°C. Levels of infection were low at 4 and 8 h of leaf wetness and continued to increase with longer wetting periods up to 36 h. A polynomial equation was developed that provided a good fit for the data (adjusted R2 = 0.93). Isolates differed in aggressiveness, but there was no significant difference among isolates in their response to temperature and leaf wetness duration

First report of Alternaria brown spot of Minneola tangelo in Turkey

Minneola tangelo, a hybrid of Dancy tangerine and Duncan grapefruit, is widely grown in Turkey for export to Europe. In 1995, 2,000 ha of diseased Minneola tangelo trees were observed in the Cukurova region. Ninety percent of orchards in this region were diseased but disease severity depended upon the location and the disease control strategies employed in each orchard. Symptoms were typical of Alternaria brown spot (1). Fruit symptoms included light brown, slightly depressed spots to circular and dark brown areas on the external surface. On leaves, symptoms ranged from small brown circular spots to irregular blighted areas with characteristic yellow halos. The apices of some young shoots were defoliated. Infected young fruit and leaves often dropped from the tree and the mature fruit were unmarketable due to lesions on the fruit. Alternaria was consistently isolated from diseased leaves and fruit on potato dextrose agar and produced dark brown mycelia and pigmented septate conidia similar to those described previously (1). Detached immature Minneola leaves were inoculated with aqueous conidial suspensions of 15 isolates and maintained in a chamber at near 100% relative humidity for 3 days at 26°C. Thirteen of these isolates caused necrotic lesions on the leaves similar to those observed in the field, i.e., dark, necrotic spots with necrosis extending outward on some veins. Alternaria was reisolated from all infected leaves. This is the first report of Alternaria brown spot of Minneola tangelo in Turkey