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frosty pod rot

Moniliophthora roreri
This information is part of a full datasheet available in the Crop Protection Compendium (CPC);www.cabi.org/cpc. For information on how to access the CPC, click here.

Distribution

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Host plants / species affected

Main hosts

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Theobroma cacao (cocoa)

List of symptoms / signs

Fruit - abnormal shape
Fruit - discoloration
Fruit - extensive mould
Fruit - lesions: black or brown
Fruit - lesions: on pods
Fruit - mummification
Seeds - rot

Symptoms

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.

Prevention and control

M. roreri only infects the fruit and only of two closely related genera, Theobroma and Herrania. Thus prevention can focus on prohibiting the transport of the pods, e.g. for transporting the recalcitrant seeds of these two species, whether they appear healthy or not, and enforcing this regulation effectively. However, as mentioned above, the pathogen’s latency period is deceiving and may tempt non-compliant behaviour. One key element to minimize infractions is education and training of front-liners (Krauss, 2010). However, even those with a vested interest, such as growers and researchers, do not always act consistent with their level of knowledge. Given the devastating losses frosty pod rot brings about, decision-makers are well-advised to invest in effective enforcement. 

Cultural Control and Sanitary Methods

Cultural control is the central pillar of any integrated frosty pod rot management. Diseased pods have to be removed from cocoa trees before the pathogen sporulates on these pods. The pathogen has a latent phase of approximately seven weeks; growers in frosty pod-affected countries tend to be able to recognize the disease one week before sporulation. This diagnostic capability dictates the required phytosanitation frequency, if sporulation is to be prevented. Although epidemiologically sound, weekly phytosanitation is cost-effective only in a few low-wage regions. Therefore Krauss et al. (2006a) suggested more effort in training farmers to recognise earlier stages of infection.

All diseased pods must be removed from the tree at intervals before sporulation begins. Cuts must be clean at the peduncle to avoid exposure of internal tissue, on which the fungus sporulates even after the pods are removed. Once cut off, all diseased pods can be left undisturbed on the ground, to allow for microbial inactivation of the spore mass if present (González et al., 1983; Aranzazu, 1987).  Mummified pods in the canopy can go through 10-14 sporulation cycles for up to 80 days (Ram 1989). Such spores can remain viable for up to nine months (Evans 1981), whereas pods on the ground decompose quickly and spores lose their viability.

In regions with a well-defined harvest peak and where incidence of frosty pod increases towards harvest, total pod removal (both healthy and diseased pods) at the end of the peak may be necessary to break the disease cycle (Porras and Sánchez, 1988). In any case, all mummified pods should be removed from the trees before the next flowering peak (Evans, 1981; Ram, 1989).

For effective phytosanitation, tree canopies in the plantation must be kept low and thinned by frequent light pruning, in order to provide ventilation and facilitate timely detection and removal of diseased pods (Barros, 1980; Evans, 1981; Suárez, 1987). There is no short-cut to cultural measures, including formation and maintenance pruning. Chemical and biological agents are being developed as supplementary management options (see below).   

Regulatory Control

Regions or countries free of frosty pod must maintain quarantine regulations, aimed not only at pod transport from infested areas but also related trash or residues that may harbour spores. Theobroma-free natural barriers, to break tree-to-tree dispersal, might be feasible. In-pod seed transport, as for research or breeding purposes, should require quarantine examinations in non-cocoa-growing regions. Vegetative propagation material should be dipped in fungicide suspension to prevent external transport of spores.

Internal quarantine is also possible, if some coco-producing regions in a country are free of frosty pod while others are infested.

For cultural frost pod control to be effective, phytosanitation also needs to be enforced in neighbouring plantations, as elimination of inoculum sources is useless if airborne inoculum continues coming in from nearby areas. This requires local phytosanitary regulations and can only be successful if no wild hosts of the pathogen exist in the adjacent forest, such as wild Sterculariaceae.

Chemical Control

Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:

Impact

Losses to the cocoa crop caused by frosty pod, in the absence of control efforts, depend on climatic conditions of each region and their relationship to crop phenology. Under continuous rainy weather (short or undefined dry season), high temperatures (20-30°C daily range or higher) and main pod-set peaks coinciding with rainy periods, losses can reach 80-90% of annual yields, as in the Atlantic region of Costa Rica (Enríquez et al., 1982; Porras and González, 1984). In regions with a well-defined dry season of 4 months or more, yield losses from frosty pod may remain around 20-30% when much of the pod-development cycle occurs in the dry season, as in the Machala region of Ecuador (Evans et al., 1977). Losses in Colombia's major cocoa regions average 30-40% over several years (Barros, 1977); overall losses in Peru have been put at 40-50%, with total loss in some areas and the subsequent abandonment of farms (Evans et al., 1998). These figures indicate a devastating impact on cocoa yields wherever M. roreri is present. The only reason M. roreri is not rated higher as a cocoa pathogen of global dimensions is its limited distribution to Latin America, i.e. absence from bulk producing regions in Africa and Asia. Should this continental barrier be breached, global cocoa supplies would be seriously threatened.