Yellow mould - Aspergillus flavus
Introduction
Aspergillus flavus Link is responsible for the disease of peanuts and corn commonly known as yellow mould. This disease does not reduce the yield, but the the quality of the produce is very poor. In the early 1960s, aflatoxin, a toxic metabolite of A. flavus was found in peanut meal. Feed prepared with this meal caused the death of 100 000 turkeys in Great Britain. A very small amount (10-20 ppb) can produce fatal liver cancer in young animals. Yellow mould is more severe in the tropics, with symptoms appearing both early in the growth of the peanut seedlings, and near harvest time on pods and seeds in the soil.
Disease Symptoms
Symptoms first appear as spots on the cotyledons of the seedlings. Seedlings and ungerminated seeds shrivel to become a dried brown to black mass covered by yellow or green spores. Plants that survive germination and emergence appear chlorotic due to the presence of aflatoxin throughout the plant. The roots are stunted and lack a secondary root system, a condition known as aflaroot. The leaves are small and pointed with a thick and leathery texture. Infected seedlings may survive infection in optimal growing conditions, then yellow mould of peanut pods and seeds may occur, especially in dry conditions.
Following harvest, further infections may develop, with fungal growth covering the seed surface and invading the seed itself. A yellow to brown discolouration, and weight loss occurs as a result.
Aspergillus flavus is commonly associated with boll rot of cotton. The fungus stains and weakens the lint fibre. Seed infection results in reduced quality and viability and the production of aflatoxin.
Description of the Pathogen
Aspergillus flavus produces hyphae that are colourless, septate and branched. A vesicle is borne at the end of each long conidiophore. On this vesicle, rows of sterigmata develop, that bear chains of yellow-green to blue-green conidia. The conidial heads, each containing a mass of conidial chains, appear as masses up to 600 um in length and 100um in diameter. Sterigmata of A. flavus are borne in single or double series on an elongate to subglobose vesicle. The radiate heads split with maturity. A. flavus may also produce sclerotia.
Geographical Distribution within Vietnam
This disease has been recorded in all parts of Vietnam.
Host Range
Yellow mould has been recorded on peanut, corn, cottonseed, coconut, pistachio nuts, and to a lesser extent on soybeans, rice, pecans, walnuts, almond and cassava.
Epidemiology
The extent of yellow mould damage and aflatoxin production is dependent on the environmental conditions and production, harvesting and storage practices.
The pathogen is seedborne and soilborne, and active in high humidity (90-98%) and low soil moisture. Temperatures conducive to growth are 17-42°C with aflatoxin production between 25-35°C.
Control
Controlling insect pests of the peanut pods may reduce the infection rate due to the lack of an entry point for infection, as with minimising damage during digging, combining and conveying. Avoiding fluctuations in seed moisture can also prevent pod damage. Selecting peanut cultivars with lignin in the seed coats will also maintain an intact pod. The use of high quality seed treated with protectant fungicides may decrease the severity of the disease.
In storage, seed must be kept at a relative humidity below 70% so that the seed moisture will be 7-9% and A. flavus growth is not favoured. Aflatoxin production in cottonseed usually occurs in the field and not in storage since seed moisture levels at harvest are quite low.
Other cultural practices designed to reduce the severity and incidence of A. flavus infection include:
- Planting early to optimise soil moisture and to avoid drought stress and insect activity.
- Plant varieties tolerant to pod rots, nematodes, insects and fungi.
- Practice crop rotation and proper fertilisation to reduce the incidence of soilborne pathogens.
- Supplement irrigation during drought to avoid plant stress.
- Harvest when a majority of pods are mature and in dry conditions.
- Transport pods in vented trucks to prevent moisture buildup.
- Dry peanut pods with moisture content above 9% using forced air.
- Manually or electronically inspect peanuts for sale and avoid the direct consumer use of mouldy peanuts.
- Destroy peanut meal containing illegal levels of aflatoxin instead of using it as animal meal.
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