Effect of Sampling Time, Quantification Method, and Tree Sample Pooling on Phytophthora cinnamomi Root Quantities in South African Avocado Orchards.

Phytophthora cinnamomi is a destructive soilborne pathogen causing Phytophthora root rot on avocados worldwide. Little is known about the effect of root sampling time, root quantification method (quantitative real-time PCR [qPCR] versus baiting), and tree sample pooling strategies on the quantification of the pathogen in roots in avocado orchard trees. This was investigated in six avocado orchards in two climatically different production regions (Mooketsi and Letaba) in the Limpopo Province, South Africa, over a 2-year period. Two different tree sample pooling strategies, consisting of either a four-pooled group (four groups each containing five pooled trees) or a single-pooled group (20 trees pooled) per 1 ha, were both shown to be suitable for quantifying P. cinnamomi in tree roots using qPCR or root baiting. P. cinnamomi root quantities from the two tree sample pooling strategies were significantly correlated for both quantification methods. Both quantification methods were suitable for quantifying the pathogen in roots, although qPCR was superior to root baiting at identifying significant differences in P. cinnamomi quantities among root sampling time points. The effect of sampling time was dependent on the investigated year. In 2017, root quantities, which were only evaluated using qPCR, did not reveal a consistent trend of a specific sampling time yielding the highest root quantities for most of the orchards. However, five of the orchards in 2018, based on the qPCR analyses, contained significantly higher P. cinnamomi root quantities in May (late autumn) than in March (early autumn), August (late winter), and October/November (late spring). In 2018, P. cinnamomi root DNA quantities were significantly positively correlated with the number of soil temperature hours at 20 to 24 and 20 to 29°C 2 months preceding the root sampling dates and negatively correlated with the number of hours at 15 to 19°C 2 months preceding root sampling. Our study has identified P. cinnamomi root quantification methods and tree sample pooling strategies, which will be useful for understanding the biology of the pathogen and when disease management strategies should be in place.

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

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.