Investigation of Vascular-Streak Dieback of Cocoa in Papua New Guinea

Abstract

Vasoular·streak diebaok of ooooa in Papua New Guinea has different symptoms from the diseases oommonly referred to as ooooa diebaok in other ooooa·growing oountries. The symptoms of this destruotive disease of ooooa have not previously been reported outside Papua New Guinea. A speoies of tulasnelloid fungus, Oncobasidium theobromae (Talbot & Keane), was oonsistently assooiated with the disease. It grew speoifically within xylem vessels of diseased stems and leaves, preoeding symptom development. It was the only fungus whioh was oonsistently isolated from, and sporulated on, living diseased ooooa stems: it grew from xylem vessels exposed on leaf soars formed by the absoission of diseased leaves and formed white, effused, adherent fruit bodies exolusively on these leaf soars and their adjaoent bark. When spores were shed from these fruit bodies on to expanding leaves of young seedlings, the disease symptoms developed after 3 months and the fungus was re-isolated. o. theobromae therefore appears to be the oause of vasoular-streak diebaok. The fungus did not sporulate in oulture but oonditions neoessary for fruit body formation and sporulation were studied in the field and fluotuations in fruit body numbers were oorrelated with fluotuations in disease inoidenoe.