Climatic distribution of citrus black spot caused by 'Phyllosticta citricarpa'. A historical analysis of disease spread in South Africa

Abstract

Citrus black spot (CBS), caused by Phyllosticta citricarpa, is one of the main fungal diseases of citrus worldwide. The Mediterranean Basin is free of the disease and thus phytosanitary measures are in place to avoid the entry of P. citricarpa in the EU territory. However, the suitability of the climates present in the Mediterranean Basin for CBS establishment and spread is debated. As a case study, an analysis of climate types and environmental variables in South Africa was conducted to identify potential associations with CBS distribution. The spread of the disease was traced and georeferenced datasets of CBS distribution and environmental variables were assembled. In 1950 CBS was still confined to areas of temperate climates with summer rainfall (Cw, Cf), but spread afterwards to neighbouring regions with markedly drier conditions. Actually, the hot arid steppe (BSh) is the predominant climate where CBS develops in South Africa nowadays. The disease was not detected in the Mediterranean-type climates Csa and Csb as defined by the Koppen-Geiger system and the more restrictive Aschmann's classification criteria. However, arid steppe (BS) climates, where CBS is prevalent in South Africa, are common in important citrus areas in the Mediterranean Basin. The most noticeable change in the environmental range occupied by CBS in South Africa was the amount and seasonality of rainfall. Due to the spread of the disease to dryer regions, the minimum annual precipitation in CBS-affected areas declined from 663 mm in 1950 to 339 mm at present. The minimum value precipitation of warmest quarter also declined from 290 to 96 mm. Strong spatial autocorrelation in CBS distribution data was detected, so further modelling efforts should consider the relative contribution of environmental variables and spatial effects to estimate the potential geographical range of CBS.