Late blight / Root rot

Phytophthora spp.

1. Identification

Phytophthora spp. is a group of pathogenic oomycetes responsible for devastating diseases in agricultural, forestry and ornamental crops. The most common diseases include root rots, wilts, collar canker and aggressive blights.

Diseases caused by Phytophthora spp.:

• Root rot of potato — caused by Phytophthora infestans (Mont.) de Bary
• Potato late blight — caused by Phytophthora infestans (Mont.) de Bary
• Citrus collar rot — caused by Phytophthora citrophthora R.E. Sm. & E.H. Sm.
• Collar rot and gummosis — caused by Phytophthora parasitica Dastur
• Avocado root rot — caused by Phytophthora cinnamomi Rands
• Trunk canker in chestnut — caused by Phytophthora cambivora (Petri) Buisman
• Cork oak decline — associated with Phytophthora cinnamomi Rands
• Root rot in ornamentals — caused by Phytophthora nicotianae Breda de Haan

These species affect horticultural, fruit, forest and ornamental crops, causing severe economic losses.

2. Causal agent

Oomycetes of the order Peronosporales, family Peronosporaceae, genus Phytophthora. They produce sporangia, motile zoospores and resistant oospores. They are soil‑borne and tissue‑infecting pathogens with high survival and dispersal capacity in humid environments.

3. Main hosts

• Vegetables: potato, tomato, pepper
• Fruit crops: citrus, avocado, chestnut
• Forest species: cork oak, chestnut, oaks
• Ornamentals: roses, rhododendrons, azaleas, camellias
• Industrial crops: tobacco, coffee

Many species have a broad host range.

4. Symptoms

Symptoms vary depending on species and host. In roots and collar, dark necrosis, rots and loss of vigour occur, leading to progressive plant decline. On leaves, water‑soaked spots, blight and sudden wilt may appear. On fruits and stems, brown lesions, gummosis and canker are observed. In crops such as potato and tomato, Phytophthora infestans causes aggressive late blight, with water‑soaked lesions that rapidly progress to necrosis, destroying foliage and tubers.

5. Disease cycle

Phytophthora spp. survives in soil, water, plant debris and infected tissues.

Dispersal:

• Zoospores transported by water
• Sporangia transported by rain
• Dispersal through irrigation
• Dispersal via tools and machinery

Infection:

• Favoured by waterlogged soils
• Occurs in roots, collar, leaves and fruits

Polycyclic cycle:

• Multiple infections throughout the season, especially under humid conditions

6. Favourable conditions

Development is favoured by wet soils, poor drainage, excessive irrigation, moderate temperatures and prolonged leaf wetness. Saturated environments promote zoospore release, increasing infection rates. Contaminated plant debris and runoff water act as continuous inoculum sources.

7. Disease management

Management focuses on improving drainage, reducing soil moisture and removing infected tissues. Resistant rootstocks and crop rotation reduce disease pressure. Regular monitoring enables timely interventions. 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 Phytophthora spp.
• FAO – Guidelines for Integrated Disease Management in Horticultural and Tree Crops.
• Erwin, D.C., & Ribeiro, O.K. (1996). Phytophthora Diseases Worldwide.