The Effects of Postharvest Treatments and Sunlight Exposure on the Reproductive Capability and Viability of <i>Phyllosticta citricarpa</i> in Citrus Black Spot Fruit Lesions

• Main Authors: Providence Moyo, Paul H. Fourie, Siyethemba L. Masikane, Régis de Oliveira Fialho, Lindokuhle C. Mamba, Wilma du Plooy, Vaughan Hattingh
• Format: Article
• Language: English
• Published: MDPI AG 2020-12-01
• Series: Plants
• Subjects: CBS; pycnidiospores; dispersal; control; fungicide; wax
• Online Access: https://www.mdpi.com/2223-7747/9/12/1813

Citrus black spot (CBS) is caused by<i> Phyllosticta citricarpa</i>, which is classified as a quarantine organism in certain countries whose concerns are that CBS-infected fruit may be a pathway for introduction of the pathogen. This study evaluated the reproductive capability and viability of <i>P</i>.<i> citricarpa</i> under simulated conditions in which the whole fruit, peel segments, or citrus pulp with CBS lesions were discarded. Naturally infected ‘Midknight’ Valencia orange and ‘Eureka’ lemon fruit, either treated using standard postharvest sanitation, fungicide, and wax coating treatments or untreated, were placed into cold storage for 5 weeks (oranges at 4 °C and lemons at 7 °C). Thereafter, treated and untreated fruit were incubated for a further 2 weeks at conditions conducive for CBS symptom expression and formation of pycnidia. The ability of pycnidia to secrete viable pycnidiospores after whole fruit and peel segments or peel pieces from citrus pulp were exposed to sunlight at warm temperatures (±28 °C) and ±75% relative humidity levels was then investigated. The combination of postharvest treatments and cold storage effectively controlled CBS latent infections (>83.6% control) and pycnidium formation (<1.4% of lesions formed pycnidia), and the wax coating completely inhibited pycnidiospore release in fruit and peel segments. Pycnidiospores were secreted only from lesions on untreated fruit and peel segments and at low levels (4.3–8.6%) from peel pieces from pulped treated fruit. However, spore release rapidly declined when exposed to sunlight conditions (1.4% and 0% after 2 and 3 days, respectively). The generally poor reproductive ability and viability of CBS fruit lesions on harvested fruit, particularly when exposed to sunlight conditions, supports the conclusion that citrus fruit without leaves is not an epidemiologically significant pathway for the entry, establishment, and spread of <i>P. citricarpa</i>.