Root rot

Pythium spp.

1. Identification

Pythium spp. is a group of soil‑borne pathogenic oomycetes responsible for aggressive diseases in seedlings, roots and the collar region of various agricultural crops. The most common diseases include damping‑off, root rot and water‑soaked rots in humid or saturated environments.

Diseases caused by Pythium spp.:

• Damping‑off of seedlings — Pythium ultimum Trow
• Aggressive root rot in cucurbits — Pythium aphanidermatum (Edson) Fitzp.
• Collar rot in warm‑climate crops — Pythium aphanidermatum (Edson) Fitzp.
• Damping‑off in nurseries — Pythium irregulare Buisman
• Sudden wilt in young plants — Pythium debaryanum Hesse
• Root rot in ornamentals — Pythium splendens Braun
• Severe water‑soaked rot in cucurbits and ginger — Pythium myriotylum Drechsler

These species affect horticultural crops, ornamentals, nurseries and intensive production systems, causing significant losses.

2. Causal agent

Oomycetes of the order Peronosporales, family Pythiaceae, genus Pythium. They produce sporangia, motile zoospores and resistant oospores. They are typical pathogens of wet soils, saturated substrates and irrigation systems.

3. Main hosts

• Vegetables: cucurbits, solanaceous crops, brassicas
• Ornamentals: roses, chrysanthemums, potted plants
• Nurseries: seedlings of multiple species
• Tropical crops: ginger, papaya, tropical squash

Many species have a broad host range.

4. Symptoms

Symptoms include damping‑off in seedlings, characterised by sudden wilt, collapse of the collar and rapid death. In established plants, water‑soaked root rot, brown necrosis and loss of vigour are observed. In cucurbits and ornamentals, severe rots may occur, leading to total plant collapse. In saturated environments, disease progression is rapid and destructive.

5. Disease cycle

Pythium spp. survives in soil, irrigation water, substrates and plant debris.

Dispersal:

• Zoospores transported by water
• Sporangia dispersed through irrigation
• Movement of contaminated substrates
• Contaminated tools and containers

Infection:

• Favoured by waterlogged soils
• Occurs in roots, collar and seedlings
• High incidence in nurseries and greenhouses

Polycyclic cycle:

• Multiple infections throughout the season, especially in humid environments

6. Favourable conditions

Development is favoured by saturated soils, poor drainage, excessive irrigation, moderate to high temperatures and high humidity. Intensive production systems, reused substrates and recirculated water increase disease pressure.

7. Disease management

Management focuses on improving drainage, reducing soil moisture and using clean substrates. Disinfection of containers, tools and irrigation systems is essential. Well‑drained substrates and controlled irrigation practices reduce incidence. Biological control may be applied with authorised antagonists. Chemical control must follow local regulations and alternate modes of action to avoid resistance.

References

• Agrios, G. N. (2005). Plant Pathology. Elsevier.
• EPPO – Diagnostic Protocols for Pythium spp.
• FAO – Guidelines for Integrated Disease Management in Horticultural Crops.
• Erwin, D.C., & Ribeiro, O.K. (1996). Phytophthora and Pythium Diseases Worldwide.