Phytophthora root rot of tomatoes
Note Number: AG0199
Sze Flett and Bill Ashcroft, Tatura
Updated: December, 1997
Importance of the disease
Root rot of tomatoes is a widespread problem affecting direct-seeded, furrow irrigated crops grown for the processing industry. Yield losses due to root rot may account for 25% of the total annual production, and losses to individual growers may be higher.
Symptoms of the disease
The above-ground symptoms of the disease include a stunted appearance, wilt and the die-back of tops (see picture). The fungus usually infects the tap roots and causes a brown rot, often located close to the crown.
In severe cases, the tap root may be cut off completely by the rot. Sometimes, a mass of fibrous roots may develop at the crown to compensate for the loss of the tap root.
When severely diseased plants are water-stressed, for example under hot, windy conditions, the tops will wilt and death may occur. The vascular tissues of diseased roots often show a brown discolouration similar to that seen in waterlogged plants.
Young seedlings may also be affected by the disease. Symptoms in seedlings include the collapse of the stem near the base of the plant.
Cause of disease
The disease is caused by a soil-borne fungus called Phytophthora nicotianae var. nicotianae, which exists as resting spores in the soil. When the soil is moist, these resting spores may develop sac-like structures known as sporangia which contain many swimming spores (zoospores).When the soil is saturated, zoospores are released and will swim to nearby tomato roots. As roots become infected and the disease develops, more sporangia develop in the infected root tissues and the cycle of zoospore release and root infection continues through the growing season.
Influence of the environment
Phytophthora nicotianae grows most rapidly at 25 to 28°C and these temperatures are frequently encountered in the field during the tomato growing season in Victoria.
Saturated soil conditions, created by irrigation or rainfall, favour the spread of the fungus because zoospores can swim in the water-filled pores between the soil particles.
Research has also shown that prolonged irrigation or waterlogging can reduce soil oxygen levels and these conditions can greatly increase root rot severity. Similarly, soils that have slow water penetration or poor drainage will also provide ideal conditions for disease.
Little is known about the survival of the fungus in soil but tomato crops are known to be affected by the disease even after a six-year rotation. In the past, many growers have managed to avoid severe disease outbreaks by long crop rotations or leasing new ground for tomatoes. However, as new land becomes scarcer, growers are forced to look for alternative control measures.
There are no practical control measures available for the disease as yet, although research has shown that certain soil applied systemic chemicals have potential as short-term treatments for root rot.
The major influence of soil water on Phytophthora nicotianae means that any measure to improve soil drainage and to minimise the period required for each irrigation will help reduce disease severity and incidence.
Avoid over-watering, particularly during early stages of crop development, because seedlings do not require as much water as mature plants and because seedlings appear to be more susceptible to severe disease. The period of furrow irrigation may be shortened by increasing the slope or fall of the paddock and by reducing the length of the run.
Practices that will improve soil drainage, such as soil amelioration with gypsum and increasing the height of beds, should also reduce the chances of severe disease outbreaks. Alternative irrigation practices such as trickle irrigation are promising options because they can provide more precise water-management.
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.