Fusarium Basal Rot

Causal Organism: Fusarium oxysporum f.sp. cepae

Crops Affected: onions and other plants in the genus Allium

Disease Cycle:

• Fungal pathogen which can cause losses in both the field and in storage
• Overwinters and lasts in the soil as tough spores (called chlamydospores)
  • Chlamydospores are spread by soil, water, or in the air
• Infection of the hose plant occurs via the roots (directly) or through wounded or weakened tissues
  • Wounding can occur due to other diseases, stress, or injury from insect pests (e.g. onion maggots)

Conditions Favouring Disease Development:

In Field

• Wounded tissues
• Temperatures between 15°C to 30°C
• High soil temperatures
• Adequate soil moisture (like that required for onion growth)

In Storage

• Temperatures between 20°C and 30°C allow for rapid disease development
• High relative humidity

Symptoms:

• Initial symptoms include tip dieback and yellowing of the leaves, which then develops further until all the foliage is yellowed and withered
• Infected plants will pull easily out of the ground and may appear to be somewhat lopsided
• The disease may affect all or part of the basal plate of the bulb
• Roots rot
• The rot on the bulb may have a pinkish-brown colour
  • White mycelium may develop on the rotted area if conditions are moist
• Symptoms may not develop in the field if infection occurs in the late season
• Cut bulbs in storage will exhibit a discoloured basal plate, starting at the outside layers and moving upward

Management:

• Maintain a rotation of at least 3 years between crops
• Ensure soils are well-drained
• Start with disease free plant material (sets)
• Keep crop stress-free
• Cure bulbs before storage to tighten necks
  • This period of curing will also allow infected bulbs to be culled out, as much as possible
• Store onions below 15°C to slow disease development
  • Long term storages should be held at 0°C, with the air relatively dry (65-70°C relative humidity)
• Some cultivars are tolerant of the disease