Citrus black spot (CBS) is caused by Phyllosticta citricarpa, 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 P. citricarpa 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 P. citricarpa.

Green mold and sour rot, caused by Penicillium digitatum and Geotrichum citri-aurantii, respectively, are the two most important citrus postharvest diseases in South Africa (SA). Management relies on postharvest fungicide application, although other practices such as minimizing injuries, using sanitizer washes, and managing temperatures of harvested fruit significantly contribute to minimizing losses. Propiconazole, a demethylation inhibitor (DMI) fungicide, was registered in SA for sour rot control in 2015 and showed efficacy against green mold, but its use poses a risk of cross-resistance in P. digitatum to the widely used imazalil, since these two fungicides have the same mode of action (inhibition of the 14α demethylase during fungal sterol biosynthesis). Thus, the aim of this study was to determine propiconazole baseline sensitivity from organic orchard populations, verify pathogen species identity, and assess propiconazole sensitivity of G. citri-aurantii and P. digitatum from South African packhouses, a juice factory, and orchards from Eastern Cape and Western Cape citrus production regions. Isolates of G. citri-aurantii and P. digitatum from three unsprayed orchards (biological production) were used to establish the baseline sensitivity in 56 isolates of each pathogen. Mycelial growth tests of 182 G. citri-aurantii and 157 P. digitatum isolates from commercial fruit were conducted at discriminatory doses of 0.9 mg/liter of propiconazole and/or 0.5 mg/liter of propiconazole. Furthermore, inoculated citrus fruit was used to establish propiconazole efficacy in planta. For G. citri-aurantii, the mean baseline EC ₅₀ was determined at 0.53 mg/liter (0.12 to 1.39 mg/liter) and for P. digitatum at 0.10 mg/liter (0.04 to 0.27) from untreated South African orchards. After 3 years of postharvest fungicide use, a high number of propiconazole-resistant P. digitatum isolates were recovered (22.2%), while only one packhouse was identified to harbor propiconazole-resistant G. citri-aurantii isolates. The resistant G. citri-aurantii isolate showed a higher mean incidence of causing sour rot at the registered propiconazole concentration (24.8%) compared with the sensitive isolate (7.0%), while a moderately sensitive P. digitatum isolate (EC ₅₀ = 0.147 mg/liter) resulted in 17.0% green mold incidence. It is important that the development of fungicide resistance be monitored, particularly following the intensive use of DMIs such as imazalil, so that fungicide resistance management strategies are put in place to prevent losing the use of the available fungicides.