Fusarium wilt of cotton, caused by the fungus Fusarium oxysporum Schlechtend. f. sp. vasinfectum (Atk.) Snyd. & Hans, was first identified in 1892 in cotton growing in sandy acid soils in Alabama (8). Although the disease was soon discovered in other major cotton-producing areas, it did not become global until the end of the next century. After its original discovery, Fusarium wilt of cotton was reported in Egypt (1902) (30), India (1908) (60), Tanzania (1954) (110), California (1959) (33), Sudan (1960) (44), Israel (1970) (27), Brazil (1978) (5), China (1981) (17), and Australia (1993) (56). In addition to a worldwide distribution, Fusarium wilt occurs in all four of the domesticated cottons, Gossypium arboretum L., G. barbadense L., G. herbaceum L., and G. hirsutum L. (4,30). Disease losses in cotton are highly variable within a country or region. In severely infested fields planted with susceptible cultivars, yield losses can be high. In California, complete crop losses in individual fields have been observed (R. M. Davis, unpublished). Disease loss estimates prepared by the National Cotton Disease Council indicate losses of over 109,000 bales (227 kg or 500 lb) in the United States in 2004 (12).

Streptomyces hygroscopicus var. geldanus controlled rhizoctonia root rot of pea in previously sterilized soil if incubated for 2 or more days prior to infesting soil with Rhizoctonia solani and planting. Streptomyces hygroscopicus also reduced saprophytic growth and the population of R. solani in soil. Growth of R. solani was inhibited by geldanamycin, an antibiotic produced by S. hygroscopicus, on nutrient media. Methanol extracts of soils in which the antagonist was incubated for 2 or more days inhibited growth of R. solani. Geldanamycin concentration was 88 μg per gram of soil after 7 days of incubation. Bioautography of soil extracts indicated that the inhibitory compounds were geldanamycin and two other compounds, also found in the geldanamycin standard. The period of incubation necessary for antibiotic production and disease control was similar, with no disease control occurring where no antibiotic was detected. Amending soil with geldanamycin, in amounts equivalent to that produced after 2 or 7 days of incubation, controlled disease and reduced saprophytic growth of the pathogen. Lesser amounts of the antibiotic did neither. No evidence for antagonism owing to competition (nitrogen, carbon) or parasitism was found. Streptomyces hygroscopicus and geldanamycin also affected plant growth.

Recently killed cover crops often interfere with crop seedling growth. Controlled‐environment and field studies were conducted to characterize the nature and persistence of cover crop interference with sorghum [Sorghum bicolor (L.) Moench] seedling growth and to test several seed‐zone management practices that might alleviate detrimental effects. Germination, root and shoot length, and disease incidence of sorghum germinated at 25°C for 5 d in soil collected 2, 4, 7, 14, 23, and 32 d after killing cover crops indicated legume cover crops were more detrimental to seedling growth than were nonlegumes. Surface residues, subsurface residues, and residue leachates contributed to the deleterious effects. Seedling shoot disease incidence of 50% persisted through 32 d when legume residues were mixed into soil or placed on top of soil at planting, but disappeared by 7 to 14 d if residues were removed. Pathogenic organisms isolated from lesions on seedlings indicated legume cover crops increased damage due to Rhizoctonia solani Kühn. In a no‐till field study, stand density was reduced 15% and aboveground seedling dry weight was reduced 45%, from 85 to 45 mg plant−1 28 d after planting, when sorghum was planted 1 d after killing crimson clover (Trifolium incarnatum L.) compared with planting 21 d or longer after killing. Insecticide, activated charcoal, or CaO2 seed coating improved sorghum stand density 15%, but did not affect seedling size. In‐furrow fungicide drench had no effect on stand density, but phytotoxic effects of the fungicide reduced shoot and root growth rates in both field and controlled‐environment studies. Residue removal combined with selected in‐furrow treatments may allow the interval between cover‐crop killing and successful no‐till planting to be reduced to less than 7 d.

The role of soilborne pathogens in flood damage on soybeans, Glycine max, was examined using six genotypes representing a reputed range of flood tolerances. Genotypes were planted in single-row plots from 1996 to 1998 with flood treatments of no flood, flood at emergence (3-day duration), or flood at the fourth leaf node growth stage (7-day duration). Three or four days after removing each flood treatment, plant stands were estimated and 15 plants were collected from each plot, weighed, and rated for root discoloration. Roots were assayed for the presence of fungi and other filamentous eukaryotic organisms. Plant stands were reduced by flooding at emergence compared with the nonflooded control. Flooding at both growth stages caused significant increases in root discoloration compared with nonflooded soybeans. Plant weights were reduced in 2 of 3 years for flooding at emergence. Pythium was the only genus of filamentous organisms whose isolation frequency increased with flooding. Of the 60 Pythium isolates evaluated representing the different cultural groups based on appearance and growth rate on potato dextrose agar, cornmeal agar, and V8 agar, 47% were moderately to highly virulent on soybean. Pythium spp. isolated from soybean included the pathogenic species P. ultimum, P. aphanidermatum, P. irregulare, and P. vexans and Group HS. Nonpathogenic P. oligandrum also was isolated from soybean.

ing a more informed seed treatment decision before planting. Also, since seed cost and associated technology The effects of fungicide seed treatments on seeding rate, location, fees have made seed cost a greater percentage of opsimulated rainfall at emergence, time of planting, and seed quality erating costs (Lambert and Lowenberg-DeBoer, 2003), were analyzed for soybean [Glycine max (L.) Merr.] in this study. Variation in plant emergence allowed estimation of economically analyses surrounding seeding rate and replanting decioptimal seeding rates and partial returns (PR ϭ Gross revenue Ϫ sions for soybean are becoming more important to pro-Seed cost) across seed treatment options. Study results proved a single ducers. seed treatment to be superior across most study conditions. In fact, In this study the effects of four different seed treata comparison of optimally treated to untreated seed revealed that a ments: Fludioxonil (Maxim), Carboxin-Tetramethylthiseemingly insignificant input in terms of cost (Ͻ$8.65 ha Ϫ1 ) enhanced uram disulfide-Metalaxyl (Stiletto), Metalaxyl (Alleprofitability by an average of $43.71 ha Ϫ1 in this study. Using high giance), and Carboxin ϩ PCNB (Vitavax ϩ PCNB) rather than low quality treated seed increased producer returns by were compared with a control with no seed treatment. 1 an average of $64.27 ha Ϫ1 . Seeding rate recommendations need to be viewed with the precaution that added seed may be low cost insurance This allowed for the assessment of the importance of against lesser-than-expected survival rates. For the cultivar Hutcheson different pathogens based on the fungicides applied to (MG V), planting in May compared with April and June provided seed (for example, Allegiance targets Pythium spp., better yields using less seed on average. Finally, as the planting season Vitavax ϩ PCNB targets Rhizoctonia solani, Maxim progressed, replanting plant population density thresholds decreased.

No abstract

Rothrock, C. S., Winters, S. A., Miller, P K., Gbur, E., Verhalen, L. M.,

The effect of flooding and Pythium ultimum on soybean, Glycine max, was determined in a series of greenhouse experiments using the cultivars Hutcheson and Archer. Seeds were planted into pasteurized soil either not infested or infested with sand-cornmeal inoculum of P. ultimum and either flooded at emergence for 2 days or at the four leaf node stage (V4) for 5 days. A nonflooded control was included in each experiment. Seeds placed directly into infested soil resulted in little or no stand for Hutcheson regardless of flood treatment, whereas stand was reduced for Archer only in the flooded infested soil treatment. Additional experiments were conducted by placing seed onto a 2- to 5-mm layer of pathogen-free soil on top of the infested soil. Flooding at emergence reduced plant height, growth stage, and top dry weight for Hutcheson and root fresh weight for both cultivars. Greater reductions for Hutcheson in root weight, and top dry weight in P. ultimum-infested soil in the soil layer experiments, also indicated that Hutcheson was more susceptible than Archer. Flooding alone decreased root weights, and infestation with P. ultimum reduced weights further resulting in an additive effect. This also was the case for plant height, growth stage, and top dry weight for Hutcheson for flooding at emergence. Root discoloration was greatly increased for both cultivars in infested soil flooded at emergence. Similar results were found when plants were flooded at V4; however, the effect was not as great as with flooding at emergence. These studies indicate that Pythium damping-off and root rot may account for a portion of the negative response of soybean to flooding. The results also indicate that Archer has some resistance to P. ultimum.