Combining Fosetyl-Al Trunk Injection or Metalaxyl Soil Drenching with Soil Application of Aldicarb for Control of Citrus Decline. H. F. Le Roux, Senior Research Pathologist, Outspan Citrus Centre, P.O. Box 28, Nelspruit 1200, Republic of South Africa. F. C. Wehner, J. M. Kotzé, and N. M. Grech. Professors, Department of Microbiology and Plant Pathology, University of Pretoria, Pretoria 0002, Republic of South Africa; and Senior Agricultural Researcher, Citrus and Subtropical Fruit Research Institute, Private Bag X112O8, Nelspruit 1200, Republic of South Africa. Plant Dis. 75:1233-1236. Accepted for publication 10 December 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-1233.

Four-year-old citrus trees growing in an orchard infested with Phytophthora nicotianae var. parasitica and Tylenchulus semipenetrans were treated every 3 mo for 2 yr with metalaxyl, fosetyl-Al, and aldicarb alone and in various combinations. Ten and 40 mo after commencement of the treatments, the disease rating of trees treated with fungicides or fungicides + aldicarb was significantly lower than that of untreated trees or trees treated with aldicarb only. The dry root mass per kilogram of soil after 40 mo was greatest in trees treated with fosetyl-Al + aldicarb. Increase in trunk diameter and canopy volume was greatest in trees treated with fungicides + aldicarb. Only trees treated with fosetyl-Al + aldicarb produced a significantly higher fruit yield than control trees harvested at 32 and 44 mo after commencement of treatments. Also, these trees yielded a consistently higher income than the control trees. At the 44-mo harvest, the net income derived from all treated trees, except those given aldicarb only, was significantly higher than that derived from control trees. Only those treatments that included metalaxyl resulted in a reduction in the incidence of P. n. parasitica in soil. The population of T. semipenetrans on roots and in soil was reduced by treatments containing aldicarb but increased again within 3 mo. None of the treatments had any significant effect on the incidence of Fusarium solani or F. oxysporum in the roots or soil.

Keyword(s): nematicides.