Inoculum Sources, Infection Periods, and Effects of Environmental Factors on Alternaria Brown Spot of Mandarin in Mediterranean Climate Conditions

Bassimba, D. D. M., Mira, J. L., and Vicent, A. 2014. Inoculum sources, infection periods, and effects of environmental factors on Alternaria brown spot of mandarin in Mediterranean climate conditions. Plant Dis. 98:409-417. Alternaria brown spot (ABS), caused by Alternaria alternata, is a serious disease affecting mandarin in humid and in semi-arid regions. The information available from Florida cannot be easily extrapolated to Mediterranean regions; thus, epidemiological studies were conducted during two consecutive years in Spain. Pathogenic isolates were found in the canopy and leaf litter and on weeds of the genus Sonchus. The pathogen survived in fallen immature leaves for up to 76 days, until complete leaf decay. Conidia of Alternaria spp. were captured continuously and pathogenic isolates were detected in all sampling dates. However, the number of pathogenic isolates was not correlated with the total captured, indicating that morphological identification is not sufficient for airborne inoculum monitoring. In contrast to humid areas, infections occurred mainly in spring and autumn. Classification tree analysis indicated that virtually all infections occurred on weeks with rainfall ≥2.5 mm and average temperature ≥12.5°C. Based on regression quantiles, the amount of rainfall, number of rain days, and total wetness duration were considered important factors increasing ABS incidence during infection periods. The development of decision support systems for ABS control in Mediterranean conditions may benefit from the restricted periods of infection and the strong influence of weather factors in disease onset. Alternaria brown spot (ABS), caused by the “tangerine pathotype” of Alternaria alternata (Fr.) Keissl., is a serious disease of some mandarin (Citrus reticulata Blanco) cultivars. The disease causes necrosis of young leaves, with severe defoliation and twig dieback. On young fruit, the disease appears as corky eruptions and necrotic depressed spots than are often associated with premature abscission and substantial yield losses. On mature fruit, necrotic spots are 1 to 10 mm in diameter and, although these lesions do not cause fruit rot, the external quality of the fruit is seriously reduced (34,36). There is no teleomorph known for the fungus and it survives as mycelia in affected plant tissues (34). The pathogen reproduces by means of conidia formed on lesions, which are disseminated by air currents and rain splash and infect susceptible tissues under adequate conditions of temperature and moisture. The pathogen produces a host-specific ACT toxin, which is selective to some mandarin cultivars such as ‘Dancy’ mandarin and its cultivated hybrids, as well as other mandarins and hybrids such as ‘Murcott’, ‘Emperor’, and ‘Ponkan’. The effect of the ACT toxin in host cells is extremely rapid, and the time required for symptom expression is very short (34, 36).