Control of Chestnut Ink Disease with Potassium Phosphite

Effectiveness of Potassium Phosphite in the Control of Chestnut Ink Disease (2009)

S. Gentile, D. Valentino, G. Tamietti

Potassium phosphite was applied to 4 year old chestnut plants either in foliar spray or by injection to control ink disease, after infecting the trees with Phytophthora cinnamomi. Spraying reduced symptoms by 90% in 30 days and later inhibited fungal colonisation of the stem. Injection fully prevented infection.

However injection fully prevented infection but its efficacy as a curative treatment depends on the level of disease severity. Potassium phosphite was injected annually into a set of old chestnut and young walnut trees infected with Phytophthora cambivora and cinnamomi respectively. The treatment was ineffective on heavily diseased trees but acted curatively on less infected ones. Two applications to the chestnuts and one to the walnuts was enough to promote plant recovery as observed by sprouts, leaf size and colour, normal fruit development, a halt to the flux of ink disease and the dehydration and compartmentalisation of disease lesions.

Field Results in Italy

In several Italian locations chestnut ink disease has resumed its activity awaking a sharp care for the life of chestnut stands. In north-west Italy the disease is caused by Phytophthora cambivora and is widespread being spread along country roads and the surface drainage system each year attacking new plants. Its control is difficult and based on agronomical measures. Trunk injections of potassium phosphite water solutions at a dose of 0.77 grams per cm of trunk diameter was very effective. On naturally infected adult chestnut trees treatment applied once a year in June/July for 3 to 5 years resulted in effective results in trees showing slight to intermediate disease severity. Improvement was noticed after the second treatment.

The treatment method was simple, cheap and free from environmental risk, allowing for the direct delivery of a proportionate quantity of fungicide to the plant. It did not show any negative side effects to the trees due to phyto-toxicity or wounding.

How Phosphite Protects Against Phytophthora

Phosphite (phosphonate) is a biodegradable fungicide that protects plants against Phytophthora dieback. Phosphite works by boosting the plant's own natural defences and thereby allowing susceptible plants to survive within Phytophthora dieback infested land.

It is important to note that there is no chemical that will eradicate Phytophthora dieback, including phosphite. However, an integrated approach can successfully control the spread and impact of the disease. An integrated approach may combine strategic phosphite treatment, controlling access, correcting drainage problems and implementing excellent hygiene protocols.

Phosphite controls many species of Phytophthora, including Phytophthora cinnamomi. Phosphite is not toxic to people or animals and its toxicity has been compared to table salt. There is a very low pollution risk associated with phosphite. When phosphite is sprayed on to the foliage of plants, it is applied at a very low rate, so any phosphite that reaches the soil is bound to the soil and does not reach the water table.

Phosphite needs to enter a plant's water transport system in order for it to be effective. This can be done by injecting phosphite into trees, or spraying the leaves. Only inject trees with a diameter at chest height of 10-14 cm or greater. Phosphite not only protects a plant from Phytophthora dieback infection, it can also help a plant to recover if it is already infected.