Growth and yield responses of soybean to aldicarb.

A series of greenhouse, phytotron, field, and microplot experiments evaluated factors that influenced plant-growth.stimulation associated with the use of the pesticide aldicarb. A phytotron experiment showed.that aldicarb increased growth, of Ransom soybean at all temperatures but was somewhat phytotoxic to Coker 156 soybean at 30 C. Soybean gave the greatest response to this nematicide at 22 C in a commercially available medium, Metromix 220. Soybean cultivars Ransom and Coker 156. exhibited increased growth in response to aldicarb or, to a lesser extent aldicarb sulfone treatments under greenhouse and microplot conditions. Enhanced soybean growth, however, did not always result in significantly greater soybean seed yield. Soil type affected soybean sensitivity to aldicarb, with. the greatest growth and yield increases generally occurring in fine-textured soils or those with high.organic matter. Plant-growth stimulation by aldicarb occurs in the absence of pests but is dependent upon concentration and edaphic and other environmental factors.

Population Changes of Heterodera glycines and Soybean Yields Resulting from Soil Treatment with Alachlor, Fenamiphos, and Ethoprop.

The population dynamics of Heterodera glycines as influenced by alachlor, fenamiphos, and ethoprop alone and in herbicide-nematicide combinations were studied in the field. Numbers of H. glycines juveniles and eggs were higher at midseason and harvest where nematicides were applied. Fenamiphos alone or in combination with alachlor provided better control of H. glycines and greater seed yields than treatments with ethoprop. Numbers of H. glycines eggs at harvest in 1980 were positively correlated with numbers of juveniles at planting in 1981 and negatively related to seed yield in 1981.

In Vitro Hatch and Survival of Heterodera glycines as Affected by Alachlor and Phenamiphos.

Heterodera glycines (race 1) eggs were exposed to aqueous solutions o f selected concentrations o f the herbicide alachlor and the organophosphate nematicide phenamiphos alone and in herbicide-nematicide combinations. Phenamiphos (0.5 mug/ml) + alachlor (0.063, 0.125, or 1.0 mug/ ml) treatments increased the incidence o f juvenile hatch over that of untreated controls at 18 days. At 18 and 25 days, phenamiphos (0.5 mug/ml) treatments contained more juveniles than did phenamiphos at 1.0 mug/ml. Phenamiphos (1.0 mug/ml) alone and in combination with alachlor (1.0 mug/ ml) suppressed hatch for 21 days and juvenile survival for more than 21 days. Alachlor treatments enhanced juvenile survival compared to the untreated control at 14 and 21 days. Technical alachlor gave results similar to those of the formulated product.

Early Growth of Soybean as Altered by Heterodera glycines, Phenamiphos and/or Alachlor.

Greenhouse and field experiments were conducted to determine the effects of phenamiphos and/or alachlor on early growth of soybean, root morphology, and infection and resurgence of Heterodera glycines (race 1). All tests were planted to 'Ransom' soybeans. In greenhouse experiments without nematodes, root growth was inhibited at 5 days by alachlor treatments and at 10 days by phenamiphos treatments; with nematodes, phenamiphos treatments enhanced root growth. Phenamiphos also suppressed early penetration of soybean roots by H. glycines in the greenhouse. Early soybean growth parameters among treatments were generally similar in the field. Nematode penetration was limited with treatments containing phenamiphos at one location. Plants treated with only alachlor had less nematode infection than did the control; however, plants treated with herbicide/nematicide combinations had more nematode penetration than did plants treated with phenamiphos alone. Alterations of root growth and interference with the efficacy of phenamiphos are two processes by which alachlor may enhance soybean susceptibility or suitability to H. glycines.