The mycelium forms a 2-3 mm thick, felt-like pseudostroma on pod lesions and on solid culture media, which is covered by a dense mat of beige to tan spores which are powdery when mature. On modified V8 medium, growth rates range from 1.3 mm per day to 6.2 mm per day, while sporulation commenced after 5.0 to 13.8 days, depending on the isolate (Philips-Mora, 2003).
The mycelium is branched, septate, with basidiomycete-like dolipore septa. It forms the pseudostroma on the surface of external as well as exposed internal necrotic lesions. The hyphae are hyaline, 2-5 µm wide, slightly constricted at the septa. Sporogenous structures are either branched or unbranched, producing chains of spores which mature basipetally. On malt extract agar, spores are mostly globose (8-15 µm diameter), sometimes slightly ellipsoidal (8-20 x 5-14 µm), with thick (1-2 µm) walls (Evans et al., 1978), whereas Philips-Mora (2003) reported somewhat lower values on modified V8 medium. There, spore shape varied between isolates, with globose spores being most common (87% to 96%) in Peruvian and some Ecuadorian isolates, whereas Costa Rican and other Ecuadorian isolates had mostly (42% to 56%) ellipsoid spores (Philips-Mora, 2003).
Given the devastating yield losses caused by frosty pod rot within a few years of disease establishment, growers in countries and regions free of the disease are anxious to maintain disease-free status and vigilantly reconfirm the absence of cream to tan pseudostromata on pods. Cocoa-producing countries particularly at risk of M. roreri introduction have developed measures for early detection and rapid response. The Dominican Republic, as the world’s largest producer of organic cocoa, linked its nation-wide diagnostic network with the incipient Caribbean Pest Diagnostic Network (Reyes Valentín et al., 2010). Brazil combined classical spore trapping with an enzyme-linked immunosorbent assay (ELISA) to detect the inconspicuous spores of M. roreri and distinguish them from other fungal taxa (Pomella et al., 2005). Sentinel stations near the borders with Colombia and Peru, where the disease is rife – far away from the cocoa-producing regions of Eastern Brazil – are proposed, in the hope that distant rapid response could win enough time to save the Brazilian cocoa industry from an invasion by M. roreri.
Early detection and a rapid response leading to eradication have never been applied successfully against frosty pod rot. After a latent infection phase of approximately seven weeks, there is an extremely narrow window of opportunity for early detection, as prolific sporulation occurs within one week of diagnostic symptom development. Thus, Krauss (2010) recommended training farmers as well as field officers in the recognition of early symptoms of frosty pod rot and offered tools for this purpose.
Where frosty pod is established, quantitative disease assessment involves the counting and removal of all pods that can be positively identified as infected with frosty pod rot. The aforementioned sliced pod assay can give diagnostic clarity.
Related invasive species
- Moniliophthora roreri
Related Farm Practice
- Light
The invasive basidiomycete pathogen M. roreri originated in Western Colombia/Ecuador. In recent years it has expanded its range in South America (Peru, Venezuela and Bolivia) and throughout Mesoamerica as far as Mexico. Africa, Asia and insular Caribbean are still free of this pathogen.
Symptoms appear only on pods, and their nature depends upon the age of the pods when infected. Pods that are infected very young 1 month) show slightly chlorotic swellings and sometimes distortion, followed by general necrosis before the pod reaches half size;the seed mass may become soft and watery.
Pods that are infected when 1-3 months old may show some swellings and/or distortion, and more generally large, necrotic, dark-brown spots with irregular borders, which grow rapidly and may cover all or part of the pod surface;larger pods show partial or total premature ripening. Necrosis spreads internally, particularly to the endocarp and placenta.
Pods that are infected after 3 months of age may show no external symptoms, or only limited necrosis, often slightly sunken, surrounded by areas of premature ripening. Infected pods are noticeably heavier than healthy ones. Internally, the endocarp, seeds and placenta may show more advanced, partial or total reddish-brown necrosis and the seed mass fails to separate from the endocarp. The pod surface remains firm in all cases.
Most of the necrotic external surface soon becomes covered by a thick, felty fungal growth (pseudostroma), at first frost-white, turning to cream, tan and then light brown. If an infected fruit is sectioned, the pseudostroma appears on the necrotic internal cut surfaces, followed by sporulation within a few days.
Infected pods remain attached to the branches and gradually shrink and dry, becoming necrotic, hard mummies, partly covered with the hardened remains of the pseudostroma.
M. roreri infects only the fruit of Theobroma and Herrania species. There is circumstantial evidence that flowers and flower cushions may harbour M. roreri, but it has never been isolated from such tissues (Evans, 1981;Ram, 1989). It has been inoculated into cocoa and other Theobroma seedlings and re-isolated from them (Evans, 1981;Ram, 1989), but no report has been found of natural infection of such tissues;it therefore does not sporulate on seedlings.