Development of an early detection method for white root disease caused by Rigidoporus microporus

Abstract

Rigidoporus microporus (Fr.) Overeem has become an increasingly important plant pathogenic fungus in Sri Lanka and in many other countries. This fungus causes white root disease in many economically important agricultural, ornamental and plantation crops such as Rubber, Tea, Coconut, Jack, Bread fruit, Mango, Cashew nuts, Carsmbola, Avacado, Cassava, Cinnamon, Cocoa, Yams, Weeping willows and Teak resulting huge losses to the growers. Moreover, the sacred “bo trees” (Ficus religiosa) and the Sri Lanka‟s national plant “Na tree”, (Mesua ferrea) are also affected. In rubber white root disease is the most destructive root disease in Sri Lankan rubber plantations and is currently spreading at an alarming rate. The disease can be controlled by drenching with systemic fungicides tebuconazole or hexaconazole. For chemical control, early disease diagnosis is important. The disease at present is identified through the foliar symptoms, yellowing and buckling. The characteristic fruit bodies are observed at later stages and by then most plants are beyond redemption. The incapability of identifying the disease during the early stages causes economic losses to the grower and they are continually confronted with this problem. The present study was aimed at developing an early detection technique enabling to overcome the above problem. The use of mulching to detect the fungal growth in artificially inoculated polybag plants is reported. The detection of the disease was by the examination of the collar region of the plants. It was revealed that till 8 weeks no plants showed any rhizomorphs on the collar region. But 10 weeks after mulching, 40% of the plants showed rhizomorphs at the collar region while there was no such growth on the infected plants without the mulch. Twelve weeks after mulching, 80% of the plants showed the fungal growth while after 14 weeks all the plants under investigation were had rhizomorphs at the collar region. The plants infected and kept without mulching did not show any rhizomorphs on the collar region till the 12th week after mulching. After 14 weeks of incubation 10% of the plants without mulching also showed foliar symptoms.. The fungal mycelium is reported to grow superficially as rhizomorphs on Hevea roots and it has also been reported that this epiphytic mycelium always grows well ahead of the area where fungal penetration occurs. This time gap can be effectively used to detect the disease early. Hence, chemical control would become more efficient as the infection is detected during very early stages. However, further investigations are necessary to assess the pathogenicity behind the rapid upward movement of the pathogen and also to demonstrate its applicability under natural field conditions.