Botanic Gardens Trust, Sydney, Australia

Department of Environment, Climate Change and Water NSW

Science



Black mould of peanut and onion - Aspergillus niger

Introduction

This fungus is responsible for Aspergillus crown, pod and seed rot of peanut and black mould of onion. This disease can be of great economic importance in warm conditions.

Disease Symptoms

Peanuts
Seedlings and young plants are more susceptible to the pathogen, and in these cases, the most obvious symptom is a sudden wilting of the young peanut plant. Diseased areas of the plant are covered in dark fungal growth. Infection of seedlings commonly occurs soon after germination . The disease progresses rapidly, and most affected plants will die within 30 days of planting.

Onions
Infected onion bulbs have a black discolouration at the neck, shallow lesions on the outer scales, streaks of black mycelium and conidia beneath the outer scales and a black discolouration in bruised areas. The entire surface of the bulb may turn black, in which case, the onion will shrivel and secondary bacteria may cause a soft rot of the bulb.

Description of the Pathogen

Upon isolation from diseased tissue, the fungus will grow on a variety of media, most optimally at 25°C, producing large black conidial heads (up to 700-800 um in diameter). These clusters of black spore heads can usually be seen without magnification.

Distribution within Vietnam

This disease has been recorded in all parts of Vietnam.

Host Range

Peanut and onion are the major hosts of this pathogen in Vietnam.

Epidemiology

This fungus is very common in the atmosphere and in the soil and is often found as a saprophyte. Infection will usually occur though damaged tissue. The disease is usually more severe on those crops grown in a continuous rotation of susceptible hosts. Warm, moist conditions favour the growth of this fungus. Adverse weather conditions, extreme fluctuations in soil moisture, poor seed quality, seedling damage by pesticides and other factors that delay seedling emergence have been shown to increase susceptibility of plants to the pathogen. Older plants are more prone to infection, especially in dry soil. Seeds may be infected causing damping-off, and the fungus is also spread on seed. Postharvest decay is favoured by humid conditions which encourage conidial development on the cut tissue.

Other crops documented as hosts of Aspergillus include cassava, taro and yam perhaps more as storage rots as these are all tuber crops. The storage organs are infected via wounds. Affected tissue has a brown discolouration, sometimes tinged with purple. Lesions remain firm until colonised by secondary bacteria. On cassava, rot may be caused by A. flavus and A. niger. On yams, A. niger is predominant and on taro, A. niger and A. flavus with the latter to a lesser extent.

Control

Fungicide seed treatments may be effective when used under conditions that promote rapid germination and emergence to prevent damping off. Post-harvest black rot can be controlled if produce is stored and transported below 15°C or under very low humidity. The disease is minimised by reducing the amount of physical damage to the storage organs.

 

mature peanut plant
Aspergillus niger symptoms on mature peanut plant 
  
young peanut plant
Aspergillus niger symptoms on young peanut plant

stem base 
Aspergillus niger stem base and root rot

culture
Aspergillus niger culture characteristics

in situ
Aspergillus niger in situ 
 

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.

Aspergillus on corn
Aspergillus flavus on corn
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