Phytophthora Root Rot

Phytophthora megasperma


On Lucerne it causes a yellow-brown soft rot on the tap root which later turns black. It may girdle the tap root and extend into the crown. Leaves turn yellow or whitish, especially the lower ones. Regrowth of affected plants after cutting or grazing is slow. Under dry conditions the plant may grow secondary surface roots from the remaining tap root allowing the plant to survive but growth is poor.

Species Affected:

Carrot, Chickpea, Crucifer vegetables, Lucerne, Melilotus, Pine trees and Soybean.


It survives in the soil as zoospores or fungal growth (mycelium) on infected plant tissue.
It is favoured by high soil temperatures (20°C) and wet, poorly drained soil conditions from periods of excessive rainfall or flood irrigation.
Irrigation water or free flowing water after heavy rain, can disperse zoospores that have been released from sporangia (spore producing sack) formed on infected roots.

Life Cycle:

Origin and History:



 Can cause rapid thinning of Lucerne stands especially if they are flood irrigated. Seedlings are very susceptible.
Can kill Chickpea seedlings.

Management and Control:

Use resistant cultivars.
Improve irrigation management and surface drainage to reduce excess water.


Treat seed with metalaxyl.
Plant tolerant cultivars. See Disease Susceptibility of Chickpea Varieties
Avoid planting on Chickpea and Lucerne stubbles.

Related and Similar Species:

Phytophthora species



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