A Change in the Symptoms of Vascular Streak Dieback of Cocoa in Southeast Asia and Melanesia
Philip KeaneDept. of Botany, La Trobe University
[email protected] Purwantara (Cocoa Research, Indonesia), Peter McMahon (La Trobe Univ) , David Guest
(Univ of Sydney), Smilja Lambert (Mars Confectionary), Josephine Saul (PNG Cocoa Res Inst), Azmi Bin Che Ahmad (Malaysian Cocoa Board)
Thank you also to Mars Inc. for financial and other support
Cocoa (Theobroma cacao)‐ domesticated in C. America and taken to SE Asia first in the 1680s and subsequently
A dieback disease of unknown cause devastated the post‐war cocoa plantings in Papua New Guinea and Malaysia (since found throughout the region & continues to be a serious diseases of cocoa; not found outside the region)
Greater VSD damage seen recently in IndonesiaDieback; death of very susceptible trees …………
The first symptoms occur much earlier, on the 2nd or 3rd flush behind the shoot tip
(i.e. on 3‐4 month‐old growth= incubation period after infection at the tip)
Typical first symptoms on 2nd or 3rd flush behind shoot tip (Papua New Guinea)
Leaves dropped off more‐or‐less as soon as they became fully chlorotic
Distinctive green islands on chlorotic leavesA strange and distinctive symptom – relatively uniform pattern of sharp green spots on fully chloroticleaves, which drop off before browning develops
Dark vascular discoloration in woodand leaf scars
Photo ,Chris Prior
Sectioning of discoloured wood, petioles and leaf veins revealed distinctive Rhizoctonia-like hyphae only in xylem vessels
White flat corticioid sporocarps form on leaf scars when leaf fall occurs during wet periods
Basidia and basidiospores of Oncobasidium theobromae
P.H.B. (Pat) Talbot’sOncobasidium theobromae
Now Ceratobasidium theobromaeSamuel et al.
Photos , Chris Prior
The fungus grows from petiole ends and leaf pieces, but can’t be cultured axenically (is an obligate parasite)‐ very different from standard ceratobasidiums
Photos, Agus Purwantara
An mysterious, new‐encounter disease ……..
Hypothesis: The pathogen co‐evolved with a plant (so far unknown) indigenous to the region, and has transferred repeatedly to cocoa wherever it has been introduced throughout the region.
Since about 2004 in Java, Sulawesi, Malaysia, Vietnam & PNG …
‐ changed symptoms: more rapid necrosis, less chlorosis, and less distinct green spots
Tip necrosis of attached leaves
Leaves stay attached longer
Full necrosis of leaves while they are still attached
Malaysia Photo, Peter McMahon
Vascular streaking and cambium browning
Vascular streaking is still evidentbut more irregular ‐ often a reduced number of very black streaks
Original symptom‐ relatively uniform vascular streaking
Different symptoms on detached petioles ……
A new position of sporulation (through cracks in petiole and main vein) because infected leaves tend to remain attached longer ……
Morphologically, the same fungus is involved – Sulawesi June 2008
Emergence of hyphae through cracks in mid‐rib and sporulationon older leaves that remainattached for extended periods
Hyphae in xylem vessel near a crack in vein
Sporulation on cracks on old, attached leaves ……
Reasons for the change in symptoms ?Change in the pathogen ? – but why has it happened in such widely
separated regions at the same time ?
Change in host genotype ? – no evidence, although possibly more VSD‐resistant types are being selected and are dominating the cocoa population ?
Change in environment ? – declining soil fertility especially K, climate change ? (but cocoa has always endured a wide range of soils & climates ?)
Another organism associated ? – some evidence of association of another Ceratobasidium spp. (a saprotroph, a common orchid mycorrhizal fungus); possibility of a symptomless virus that has built up in the cocoa population in the region ? (John Randles)
Thank you
Mars Inc.
Resistance still evident in ACIAR Trial, Aug 2009 ……TR‐01 (>20 infected shoots/tree) and Geni‐J (<3 infections/tree)