VIEW ARTICLE

Ecology and Epidemiology

Effects of Inoculum Density and Placement on Fusarium Root Rot of Peas. C. M. Rush, Agricultural Research Service, U.S. Department of Agriculture, Irrigated Agriculture Research and Extension Center, P.O. Box 30, Prosser, WA 99350, Present address: Texas Agricultural Experiment Station, 6500 Amarillo Blvd., Amarillo 79106; J. M. Kraft, Agricultural Research Service, U.S. Department of Agriculture, Irrigated Agriculture Research and Extension Center, P.O. Box 30, Prosser, WA 99350. Phytopathology 76:1325-1329. Accepted for publication 17 June 1986. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1986. DOI: 10.1094/Phyto-76-1325.

Pea roots became infected when inoculum of Fusarium solani f. sp. pisi was placed in the lower 10 cm of 30-cm containers, but no disease symptoms appeared. A high inoculum density of 5,000 colony-forming units (cfu) per gram of soil, placed in the lower 10 cm, failed to cause any measurable plant stress, but when inoculum was placed in the upper 10 cm or mixed throughout the containers, dry top and root weight, plant height, and leaf area were all significantly lower than control plants growing in uninfested soil. Plants growing in the high inoculum density soil (5,000 cfu per gram of soil) exhibited severe root rot symptoms earlier than plants growing in soil infested with 100 cfu per gram of soil. There was no significant difference between plant stress measurements whether inoculum was mixed throughout the container or placed in the upper 10 cm. Top weight, root weight, rate of node appearance, leaf area, and plant height were measured to evaluate the effect of Fusarium root rot on plant growth and development. Leaf area was the most sensitive indicator of plant stress. As early as 21 days after emergence, control plants had significantly greater leaf area than plants growing in infested soil. This study suggests that, in the absence of other stress factors, inoculum of F. s. f. sp. pisi deep in the soil has no detrimental effect on pea growth and development up to the time of flowering, when the upper 20 cm of the root system is free from infection.