Phytophthora palmivora (Butl.) Butl.; P. meadii McRae; P. botryosa Chee

Distribution

Abnormal leaf fall and leaf wither have been reported in most rubber growing countries. In the 1998 Survey the condition has been reported from most rubber producing countries as being: "the disease occurs sporadically in the country, but moderate severity occurrences are localised and in limited areas. Chemical control is usually unnecessary, but is occasionally required." In an earlier survey it was reported that in China, India and Sri Lanka the disease is a significant annual problem, whereas in Cote d'Ivoire, Malaysia, Nigeria and Thailand it is sporadic. Active, regular control measures were regarded as essential. Results of 1998 Survey.

Symptoms

Abnormal leaf fall affects mature leaves. The fungus attacks the petioles causing the leaves to shed while they are still green.The fallen leaves exhibit one or more dark brown lesions with one or two white spots of coagulated latex in the middle of each, usually towards the base of the petiole. Sometimes the leaf blades are affected with initially translucent water-soaked lesions which, in course of time, develop various colours between brown and black. When lesions are present, they are concentrated more towards the apex and margin of leaflets. Without petiole infection, the affected leaflet changes colour from green to shades of yellow and red before being shed.

Green fruit pods in all stages of development are also attacked by the fungus producing a downy white growth on the surface. The affected pods do not absciss, but remain on the tree blackened, mummified and undehisced. Terminal shoots may be attacked, resulting in dieback of green shoots.

Another symptom of foliage attack by Phytophthora observed in Brazil is leaf wither.The disease differs from abnormal leaf fall in that the fungus attacks the green shoots rather than the leaves. Young leaves wither and remain attached to the dead or dying shoot, although the leaves themselves are not attacked.

Spread

Abnormal leaf fall occurs during periods of prolonged wet weather with cool overcast humid conditions which continue for several days with little or no sunshine. Infection is by motile zoospores produced from sporangia during the active phase of the disease. These zoospores are carried in water droplets by wind or rainsplash. Green pods are generally the first to be infected providing abundant inoculum for leaf infections. The survival of the fungus from one season to another results from the existence in old infected pods, shoots, bark or in debris in the soil of two other spore forms: oospores and chlamydospores.

Disease control

The disease can be prevented by one spray of an oil-based copper fungicide before the onset of the rainy season. For aerial application, the application rate is 6kg a.i. copper oxychloride in 35 litres oil/ha. With minimicron sprayers, the rate is 3.5kg a.i. copper oxychloride in 25 litres oil/ha.

In India, the use of annual pre-monsoon spraying to control abnormal leaf fall has increased yields of rubber by up to 50 per cent.

Original data sheet: May 1994