Phytophthora Boll Rot of Cotton

S.J. Allen and Karyl-Lee WestDepartment of Agriculture, N.S.W.Narrabri Agricultural Research StationMyall Vale, Narrabri, N.S.W. 2390

A boll rot of cotton occurred in seed productionand commercial crops growing near BurrenJunction in New South Wales and St George inQueensland during March, 1985. Infected bollsbecame blackened, opened prematurely and theenclosed lint was discoloured and remainedcompact (Fig. 1). The disease was most severe ina field of the cultivar Siokra growing at St Georgewhere 29 per cent of bolls were found to beinfected.

Phytophthora nicotianae Breda de Haan var.parasitica (Dastur) Waterh. was isolated fromdiseased bolls and grown in pure culture (DAR51592 from St George). A zoospore suspension ofthe fungus was prepared by incubating sporangiain sterile distilled water for 24 hours at roomtemperature. Healthy bolls on glasshouse grownplants were sprayed with the suspension andenclosed in plastic bags. The plastic bags wereremoved after two days and symptoms similar tothose observed in the field developed withinseven to ten days. The pathogen was successfullyre-isolated thus fulfilling Koch's postulates andproving pathogenicity. This is the first record of aspecies of Phytophthora causing a boll rot ofcotton in Australia.

At St George the disease appeared seven toten days after a heavy fall of rain which occurredduring the afternoon of 7th March, 1985. Theincidence of boll rot was higher in fields of cottonthat had been furrow irrigated during the weekprior to the rain than in crops with dry soilconditions at the time of the rain. Boll rot wasmost severe in a field where cotton had beengrown each year since 1975. These field obser­vations are consistent with the reported etiologyof P. nicotianae var. parasitica on other crops.The fungus is a soil-inhabiting water mould whichreq uires water for zoospore production. Thesespores are initially produced in wet soil beneathp.ants and are splashed up on to lower leaves andfruiting structures during heavy rain (3).

P. nicotianae var. parasitica has a wide hostrange which includes safflower (Carthamustinctorius.), passionfruit (Passiflora edulis Sims.),strawberry (Fragaria x ananassa Duchesne),pineapple (Ananas eomosus [L.] Merr.), tomato(Lycopersicon eseulentum Mil!.) and citrus (Citruslimon [L.] Burm.f.: Citrus paradisi Macf.: Citrusreticutete Blanco: Citrus sinensis Osbeck) (2,3).

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Fig. 1. Symptoms of Phytophthora nieotianae var.parasitica on bolls of cotton.

The pathogen also occurs on a large number ofornamental plants many of which are derivedfrom the Australian native flora (1).

Acknowledgements

The authors thank the staff of Cotton SeedDistributors Pty Ltd of Wee Waa for theirassistance and cooperation in inspecting crops inthe St George area of Queensland and Mr JohnWalker of the Biological and Chemical ResearchInstitute at Rydalmere, Sydney for identifying thepathogen.

Financial support from the Cotton ResearchCommittee is gratefully acknowledged.

References

(1) Forsberg, L.1. (1985) - Phytophthora Species onOrnamental Plants in Queensland. AustralianPlant Pathology 14(1) : 18-20.

(2) Simmonds. J.H. (1966) - Host Index of PlantDiseases in Queensland. QueenslandDepartment of Primary Industries, Brisbane.

(3) Vock, N.T. (1978) - A Handbook of Plant Diseasesin Colour. Queensland Department of PrimaryIndustries. Brisbane.

Australasian Plant Pathology Vol. 15 (2) 1986