Getting to know your citrus postharvest diseases : Dec 2020 / Jan 2021
Postharvest cosmetic pathogens are diseases that only affect the rind or surface of the fruit. The symptoms may be severe but the internal quality is usually unaffected. The only reason we care about cosmetic pathogens is because consumers buy with their eyes, and in some cases, cosmetic pathogens may pose a phytosanitary risk to the importing country. The most important cosmetic pathogens to South African citrus packers are sooty mould, storage mould and citrus black spot (CBS). Catherine Savage, Wilma du Plooy and Lindokuhle Mamba
Next: Getting to know your postharvest diseases: brown rot
Sooty mould – the problem
Have you ever seen fruit that looks like it’s been rolled through a coal mine? That’s sooty mould (Fig 1). But this black fungus doesn’t actually grow on the citrus rind. It is really growing on the sticky honeydew deposited by insects on the rind of the fruit. The result is a sticky, black mess that can be tough to remove, particularly in any creases around the calyx. Severe sooty mould can be a problem when it covers the leaves and affects the tree’s ability to photosynthesise (Fig 2). Additionally, sooty mould covered fruit may not colour up properly.
Sooty mould – the solution
Sooty mould control also happens in the orchard, although it is an indirect control method. Honeydew secreting pests, mainly aphids, scale and mealybug, need to be controlled and that way you can reduce sooty mould. However, if sooty mould is still present on the fruit at harvest then
a high pressure spray wash over brushes can wash the fruit clean. Note that if
the disease is severe, a wash won’t be sufﬁcient. Some soaps can help loosen the mould but none are yet registered for use in SA.
Storage mould – the problem
Most packhouses and exporters are familiar with storage mould, as this decay has become more and more of a problem of late. The causal pathogen is still under debate but investigations are underway. Storage mould is really easy to identify, as it is a unique white to grey ﬂuffy growth, found only on the stem and calyx of the fruit. It does not spread to the rest of the fruit and can be wiped off really easily. But unfortunately, the pathogen is inside the decayed tissue of the stem and sepals so, if you wipe or wash it off, it grows back a few days later. Storage mould is mostly seen on lemons (Fig 3) but there are reports of storage mould on other citrus types, too (Fig 4).
Storage mould – the solution
Storage mould is a big problem, as there is nothing that we know of that can be done to stop this rot. Preliminary investigations have shown that storage mould is worse moisture build-up during transport and storage. Older stems, in conjunction with excess condensation are believed to be two contributing factors to storage mould.
Citrus black spot (CBS) –
the problem Citrus black spot (CBS) presents a major challenge for us, despite not actually affecting the taste or quality of the fruit. This pathogen can negatively affect tree health (in production regions with a climate where the disease can thrive, and if not effectively controlled) and cause black to red lesions (spots) on the leaves and fruit. Identifying CBS can be difficult, as new lesions can develop over time and they can look very different (Fig 4 and 5). Complicating the matter further is the fact that other pre-harvest pathogens can create spots that look identical. For confirmation of the disease, it is best to have an expert examine the lesion and, if needed, get the lesion tested at CRI’s diagnostic centre in Nelspruit (CRIDC@cri.co.za).
Citrus black spot (CBS) –
the solution Effective control of CBS relies on pre-harvest control. A proper spray programme with the right chemicals and at the right time is essential. The EU requires complete absence of symptomatic fruit, so the presence of even very low levels of symptomatic fruit can be problematic. On a positive note, postharvest treatments have been shown to reduce the formation of new lesions. So, if your fruit is CBS-free at packing and your spray programmes have been perfect, no new lesions should appear in the market. Contact your local extension officer or a CRI researcher if you are unsure of a CBS spray programme (firstname.lastname@example.org).