Pink Root is caused by the soil-borne fungal pathogen, Phoma terrestris (syn. Pyrenochaeta terrestris), and is a serious problem for multiple onion growers. This disease can cause significant reductions in crop yields and quality in many onion growing regions of South Africa.
Symptoms:
The most recognizable symptom of pink root is the discoloration of infected roots:
- Initial stage: Roots turn light pink (Figure 1A).
- Progression: The pink coloration intensifies, shifting to red and eventually deep purple-brown (Figure 1B).
Final stage: Affected roots shrivel, disintegrate, and die.
Figure 1. (A) the initial stages of a pink root infection; (B) deep purple and shriveled up roots from a pink root susceptible cultivar.
Plants with severe infections exhibit signs of nutrient and water deficiencies, including:
- Leaf tip dieback: Leaves turn white, yellow, or brown at the tips and eventually die (Figures 2 and 3).
- Reduced growth: Leaf number and size are diminished. Severely infected plants may bulb prematurely, producing undersized and lower-quality bulbs with reduced market value.
- Weak root systems: Infected plants are easily uprooted.
Figure 2. Leaf tip dieback/necrosis (indicated by red arrows) are secondary symptoms of a pink root infection.
Figure 3. Leaf tip dieback on pink root susceptible and resistant cultivars from an infected field – note the degree of tip dieback on the susceptible (yellow leaf tips) and resistant (green leaves) cultivars. The resistant cultivar (only three beds) used in this field was Sakata’s F1 hybrid, Hoshi.
Disease development and infection:
The pathogen is widespread and survives in the soil as dormant spores or in diseased plant debris for years. Key points about its development include:
- Spread: It moves through soil or surface water.
- Infection: Onion roots are infected upon contact with the fungus.
- Environmental conditions: Pink root thrives at soil temperatures of 24–28°C but is less active below 16°C.
The pathogen infects new roots as they develop, gradually depleting healthy roots (Figure 4). However, the disease does not persist in stored bulbs.
Figure 4. Suppressed and shrivelled roots from a susceptible onion cultivar and healthy roots from Sakata’s pink root resistant cultivar, Hoshi.
 Prevention and management
Pink root is challenging to control, but the following strategies can help minimize its impact:
1. Plant resistant varieties
Varieties with high resistance to pink root should be used in infected soils.
Sakata offers short- and intermediate-day onion hybrids like Hoshi, which exhibit high resistance to pink root and basal rot (Figure 5).
Note: Resistance may be compromised if soil temperatures exceed 28°C.
2. Optimise sowing and growing conditions
Schedule sowing so root growth occurs before temperatures reach the pathogen’s optimal range.
Use well-drained, fertile soil to promote plant health.
3. Crop rotation:
Implement a 4–6-year rotation with non-host crops (e.g., cereals) to reduce soilborne inoculum.
4. Soil management:
Apply soil solarisation or fumigation to control the fungus.
Remove infected plants and debris to limit disease spread.Â
5. Monitor environmental conditions:
Avoid overwatering, as excess soil moisture can exacerbate disease development.
Figure 5. Hoshi F1 Hybrid Onion
Sakata’s F1 hybrid onions
The short-day onion hybrid Hoshi offers:
- Disease resistance: High tolerance to pink root and basal rot.
- External qualities: Uniform, firm, globe-shaped bulbs with refined necks and attractive straw to golden-brown scales.
Disclaimer
This information is based on observations and sources believed to be reliable. However, crop performance depends on the interaction of genetic potential, seed characteristics, and environmental factors, including management practices. Sakata Seed Southern Africa (Pty) Ltd provides no express or implied warranty for crop performance based on this information and accepts no liability for losses resulting from its use. Please refer to Sakata Seed