post harvest grains and legumes

8/19/2019 Post Harvest Grains and Legumes

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Postharvest diseases – grains and

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Postharvest diseases of grains andlegumes


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Grain moulds in the tropics

Grain moulds are fungi that grow on or inseeds

• Field fungi

– request a moisture content in equilibrium withrelative humidities of 90-100%– specialized pathogens that invade developing or

mature grains, which may or may not cause

visible damage– unspecialized highly pathogenic fungi that

invade developing grain

– unspecialized primarily saprophytic fungi thatinvade moist mature grain


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• Storage fungi– request a moisture content in equilibrium with

relative humidities of 70-90%

– invade grain primarily during storage– mainly several species of Aspergillus and

Penicillium

– all primarily saprophytic– may produce mycotoxins


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Storage fungi• Effect on grains

– kill and discolour grain embryos– reduce seed quality– change chemical composition

– contaminate grains with mycotoxins• Most critical factor affecting development is

moisture

• Control of storage fungi– maintenance of the grain at low moisture

content and/or temperature

– protect from insect infestation


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Field fungi

• Maize– grown on a large scale in wet lowland and

highland tropics

– cause cob rot, kernel discolouration, loss ofviability, mycotoxin contamination, subsequentseedling blights

– stalk rots, leaf damage, insect and bird damageincrease the problem of cob and kernel rot

– drought stress and grain damage by insects

increase Aspergillus incidence in the field


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• Rice

– particularly important in lowland humid tropics– warm wet weather during flowering and grain-filling

– developing and mature grains are vulnerable tomany fungi

– cause grain discolouration, destruction, loss ofviability, mycotoxin contaminatin, seedling

mortality– several of the major rice pathogens are seed-

borne (blast, brown spot, bakanae)


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• wheat

- unspecialized fungi cause black pointdisease and head blight

- the specialized fungus Neovossia indicacauses karnal bunt; it transforms kernsinto black spore masses


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• pearl millet

– the most drought-resistant of the major tropical cereals– not likely to be exposed to prolonged wet weather after

flowering

• groundnut– of particular importance in the semi-arid tropics– most serious problem is contamination of groundnut

seeds with aflatoxins produced by Aspergillus flavus

– shells are commonly contaminated with fungi, but seedsare usually free from infection at optimal time forharvest; postharvest invasion of seeds occur if

environmental conditions do not favour rapid drying


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Aspergillus flavus on groundnut


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• Pulses– cowpea, pigeon pea, chickpea, green gram, black

gram, field beans– vital sources of protein in developing countries

– pods and seeds are susceptible to damage when

crops mature under humid conditions– pod-sucking and pod-boring insects are a major

problem


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• Control of field fungi– cultivar characteristics

• traditional cultivars• photoperiod sensitivity

• grain ripening in dry weather at the end of the rainyseason

• susceptibility to insect damage

• physical and chemical characteristics of the grain

– cropping practices

• Timely harvest


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Seed deterioration

• Fungal pathogens– Effect on seed germination: mainly storage

fungi

– discolouration of seeds and abnormalities: fieldand storage fungi

– seed quality• carbohydrate degradation

• protein degradation

• lipid degradation

– Contamination with mycotoxins


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Mycotoxins


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Important mycotoxin producers

• Aspergillus and Penicillium– produce toxins mostly in stored seeds and hay– also produce toxins on commercially processed

foods and feets (meats, cheeses, spices)– infection of seeds usually takes place in the

field

• Fusarium– produces toxins primarily on corn and other

grains

– infection can occur in the field or after storage


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Ergotism

• Oldest known mycotoxicosis• caused by several toxic substances in the

sclerotia (ergots) of the ergot fungusClaviceps

• Claviceps can contaminate rye, barley,

sorghum, millet, wheat and wild grasses• symtomps:gangrene of body extremities;

hallucinations (cfr. LSD)


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Claviceps


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Ergotism


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Aflatoxins

• produced by Aspergillus flavus and several otherspecies of Aspergillus

• produced in infected cereal seeds and mostlegumes, mostly at nontoxic concentrations (about50 ppb)

• produced at high concentrations (up to 1000 ppbor more) in peanuts, brazil nuts and other seeds ornuts grown in warm and humid regions

• can cause chronic or acute mycotoxicoses inhumans and domestic animals


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Aspergillus flavus


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Fusarium toxins

• Produced by several species of Fusarium

• groups: zearalenones, trichothecenes, fumonisins• primarily produced in molded corn• Zearalenones

– estrogenic syndrome in swine

• Trichothecins– inactivity; degeneration of cells of the bone marrow,

lymph nodes and intestines; diarrhea; bleeding; and deathin swines, cows, chicks, and lambs

• Fumonisins– neurotoxic disease of horses (blind staggers); pulmonary

edema in swine; esophageal cancer in humans


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Fusarium on corn


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Further reading

• Christensen CM (1965) Deterioration of storedgrains by fungi. Annual Review of Phytopathology3: 69-84

• Williams RJ and McDonald D (1983) Grain molds inthe tropics: Problems and importance. AnnualReview of Phytopathology 21: 153-178

post harvest grains and legumes

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