No abstract

Peanut peg strength, which influences the proportion of peanut p o d s harvested, was measured as a function of the force required either to detach the peg from the pod or to break the peg. Pegs from 10 commercial cultivars and 30 plant introductions were tested for peg strength with an Instron Test Instrument. The mean peg detachment forces for Florunner

No abstract

Peanut (Arachis hypogaea L.) cultivars resistant to Pythium myriotylum Drechs. and Rhizoctonia solani Kuhn, are needed for effective management of pod rotting diseases. The low efficiency of field screening for disease reaction in breeding for resistance has emphasized the need for improved evaluation methods. Anatomical examinations of roots, stems, leaves, pegs, and shells were made of six genotypes (TxAG-3, PI 341885, Toalson, Starr, Florunner, and Goldin I) with various degrees of resistance to pod rots in search of traits that might be used effectively in screening for disease reaction. The palisade mesophyll cells of 50-day old plants were arranged more compactly in pod rot resistant than in susceptible genotypes. An index representing total width (pm) of palisade cells/mm leaf blade was more discriminative in distinguishing among genotypes than average of either cell width or cell number alone. The distribution of lignin in peanut shells was correlated with pod rot resistance. The cell walls in the epicarp and sclerenchymatous mesocarp were thicker and more lignified in the resistant than in the susceptible genotypes. Genotypic digerences in lignin distribution were readily apparent at lOOx when shell sections were stained with phloroglucinol. Associations between anatomical traits of stems, pegs, roots, or juvenile plant leaflets and field pod rot reaction were not consistent among all genotypes. However, lignindistribution in pods, and an index representing Fm of palisade cells/mm of leaf blade individually or in combination, might be used effectively to supplement field evaluations in screening breeding lines for pod disease reaction.'Contribution from the Texas Agric. Exp. Stn., Texas A&M Univ., College Station, TA No. 20169. Part of a Ph. D. dissertation submitted by the senior author. Partial support provided as a graduate scholarship by EMBRAPA -Empresa Brasileiria de Pesquisa Agropecuaria.

Web blotch, caused by Phorna arachidicola, has varied in severity from year to year in Texas, depending on survival of inoculum on peanut crop residue, presence of susceptible peanut cultivars and suitable environmental conditions, especially favorable temperature and high relative humidity. The web and blotch symptoms may develop in sequence on the same leaflet or independently on different leaflets. At relative humidities below 80% within the plant canopy, the fungal hyphae grew under the leaf cuticle causing a web-type symptom. At relative humidities above 908, hyphal strands grew more repidly and branched extensively in the l e d resulting in the blotch-type symptom. P. arachidicola isolates from South Africa, Argentina, and the USA were ail pathogenic and caused similar symptoms. Of 32 legumes, peanut, sweetclover, hairy vetch and alfalfa were the most susceptible. Symptoms on hosts other than peanuts consisted of small necrotic spots. A hypersensitive reaction developed on eight legumes. Typical web blotch symptoms were observed only on peanuts in the field.