Inoculum Densities of Fusarium oxysporum f. sp. vasinfectum and Meloidogyne incognita in Relation to the Development of Fusarium Wilt and the Phenology of Cotton Plants (Gossypium hirsutum).

ABSTRACT Development of Fusarium wilt in upland cotton (Gossypium hirsutum) usually requires infections of plants by both Meloidogyne incognita and Fusarium oxysporum f. sp. vasinfectum. In this study, the soil densities of M. incognita and F. oxysporum f. sp. vasinfectum and the incidence of Fusarium wilt in three field sites were determined in 1982-1984. Multiple regression analysis of percent incidence of Fusarium wilt symptoms on population densities of M. incognita and F. oxysporum f. sp. vasinfectum yielded a significant fit (R (2) = 0.64) only on F. oxysporum f. sp. vasinfectum. Significant t-values for slope were also obtained for the interaction of M. incognita and F. oxysporum f. sp. vasinfectum, but densities of M. incognita and F. oxysporum f. sp. vasinfectum were also related on a log(10) scale. The physiological time of appearance of first foliar symptoms of Fusarium wilt, based on a degree-days threshold of 11.9 degrees C (53.5 degrees F), was used as a basis for determining disease progress curves and the phenology of cotton plant growth and development. Effects of Fusarium wilt on plant height and boll set were determined in three successive years. Increases in both of these plant characteristics decreased or stopped before foliar symptoms were apparent. Seed cotton yields of plant cohorts that developed foliar wilt symptoms early in the season (before 2,000 F degree-days) were variable but not much different in these years. This contrasted with cohorts of plants that first showed foliar symptoms late in the season (after 2,400 F degree-days) and cohorts of plants that showed no foliar symptoms of wilt. Regression analyses for 1982-1984 indicated moderate to weak correlations (r = 0.16-0.74) of the time of appearance of the first foliar symptoms and seed cotton yields.

Effects of Verticillium dahliae Infection of Cotton Plants (Gossypium hirsutum) on Potassium Levels in Leaf Petioles.

Two isolates of Verticillium dahliae, a black microsclerotial isolate and an isolate from potassium deficient cotton plants that forms white colonies on agar media, were examined for their effects on the potassium content of cotton plants. The potassium content of petioles from fully expanded leaves collected at random from branches 6 to 7 nodes below the terminal node were monitored during July and August in 1993 to 1995. Potassium contents of petioles from plants inoculated with V. dahliae did not differ significantly from plants injected with sterile water until the plants were nearing peak boll load. Both isolates caused a gradual development of potassium deficiency symptoms in leaves of inoculated plants and a decrease in petiole potassium, often accompanied by chlorosis and necrosis typical of Verticillium wilt. These results suggest that infection of cotton plants by V. dahliae causes an impairment in the uptake and translocation of potassium that is often associated with the development of potassium deficiency symptoms in leaves of plants with large boll loads.

Effect of combining soil solarization with certain nematicides on target and nontarget organisms and plant growth.

Field experiments compared pesticidal and plant growth effects of soil solarization, alone and in combination, with overall applications of several nematicides. Nematodes, including Meloidogyne incognita J2, that were targeted for control were significantly reduced (P < 0.05) by solarization, 1,3-dichloropropene (44 and 132 liter/ha), ethoprop (13.5 kg/ha), metham sodium (64 liter/ha), formaldehyde (111 liter/ha), and by solarization-nematicide combinations. Control of Pythium ultimum also was obtained by all of the treatments; however, none of the chemicals or combinations of chemicals and solarization controlled nematodes or P. ultimum significantly better than solarization alone. Numbers of cotton (Gossypium hirsutum cv. Acala SJ-2) seed-applied Trichoderma viride and Bacillus subtilis which colonized the plant rhizosphere were not affected. Yield of carrot and survival of cotton seedlings was sometimes increased by solarization and (or) chemical treatments. No significant phytotoxicity from soil treatments was found on cotton or carrot.

Hypersensitivity reaction of Brassica nigra L. (Cruciferae) kills eggs of Pieris butterflies (Lepidoptera: Pieridae).

Some individual plants of the mustard Brassica nigra in lowland California kill eggs of the Crucifer-specialist herbivores Pieris rapae and P. napi by producing a necrotic zone at the base of the egg, thereby apparently desicating it. This is a typical hypersensitivity reaction, but to an atypical stimulus. The eggs can be "rescued" by maintaining them in a saturated atmosphere. Attempts to demonstrate a bacterial or fungal agent associated with the reaction were unsuccessful.

Long-term storage of plant-pathogenic bacteria in sterile distilled water.

This study was made to determine the effectiveness of the preservation of plant-pathogenic bacteria in sterile distilled water. After 20 or 24 years of storage in distilled water, a very high percentage (90 to 92%) of the isolates of Agrobacterium tumefaciens and Pseudomonas spp. were still alive. Moreover, 12 of 13 viable (after 24 years) isolates of P. syringae subsp. syringae maintained their ability to produce syringomycin and were pathogenic to bean seedlings. The water-stored cells of two isolates of P. syringae subsp. syringae, when observed by electron microscopy, were smaller than cells of 24-h-old subcultures of bacterial cells grown in nutrient broth; the water-stored cells appeared plasmolysed with an electron-dense cytoplasm and thickened cell wall.

Growth Regulator Changes in Cotton Associated with Defoliation Caused by Verticillium albo-atrum.

Cotton plants, variety Acala 4-42 family 77 (Gossypium hirsutum L.,), were stem puncture-inoculated with either a defoliating isolate (T9) or a nondefoliating isolate (SS4) of Verticillium albo-atrum (Reinke and Berth.). As symptoms developed, growth regulators were assayed in diseased plants to discern their importance in the disease syndrome.An Avena coleoptile straight growth bioassay demonstrated the presence of several growth-regulatory compounds in cotton tissue extracts. Indoleacetic acid was among the compounds whose effects on coleoptile growth were influenced by disease development. Coleoptile growth due to indoleacetic acid was greater in extracts of diseased stems and leaves than in extracts of comparable healthy tissues. During the defoliation period the T9 and SS4 isolates appeared equally effective in increasing indoleacetic acid and reducing indoleacetic acid decarboxylation. Preceding defoliation, however, in plants showing equivalent symptoms the degradation of auxin was reduced more by infection with T9, the defoliating isolate. The reduced auxin degradation appeared to be releated to concomitant increases in caffeic acid and other indoleacetic acid-oxidase inhibitors in the affected tissues.Abscisic acid in tissue extracts strongly inhibited coleoptile growth. During the defoliation period gas-liquid chromatographic and ultraviolet absorption measurements revealed that abscisic acid levels were approximately doubled in T9-infected leaves but were relatively unaffected in leaves infected with the nondefoliating isolate and in stems infected with either isolate.The onset of epinasty and especially defoliation was also accompanied by increased ethylene production in diseased plants. Ethylene in gas samples taken from jars confining plants infected with SS4 or T9, respectively, was increased 2- and 5-fold over uninoculated controls. Ethylene supplied exogenously to healthy plants in concentrations as low as 0.2 microliter per liter induced both the epinasty and defoliation symptoms characteristic of Verticillium infection. Ethylene treatment did not, however, induce other symptoms of Verticillium infection and did not affect endogenous levels of abscisic acid.Defoliation of T9-but not SS4-infected plants apparently is related to the differential alterations in abscisic acid and ethylene levels induced by each isolate, and perhaps to differential alterations in initial rates of indoleacetic acid decarboxylation. These growth regulator alterations apparently are reflections of altered host metabolism rather than direct contributions of the invading fungus.

The influence of syringomycin on ribonucleic acid synthesis.

Syringomycin, a wide-spectrum antibiotic produced by strains of Pseudomonas syringae which cause bacterial canker of peach, was able to bind to salmon sperm and calf thymus deoxyribonucleic acid but not to calf thymus histone; it also inhibited ribonucleic acid polymerase activity. These abilities to bind to deoxyribonucleic acid and to inhibit ribonucleic acid polymerase were inactivated when the phytotoxic and antibiotic properties of syringomycin were inactivated.

Ethylene-induced Isocoumarin Formation in Carrot Root Tissue.

The concentrations of 3-methyl-6-methoxy-8-hydroxy-3,4-dihydroisocoumarin (MMHD) formed in carrot roots inoculated with certain fungi or treated with indole-3-acetic acid, 2,4-dichlorophenoxyacetic acid, or 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), were related to the amount of ethylene produced by the root tissue. Ethylene applied exogenously in concentrations above 0.3 ppm induced the formation of MMHD in carrot root discs. Continued production of MMHD required the continued presence of ethylene. The amounts of MMHD in the discs were reduced by CO(2), an inhibitor of ethylene action, and by reduction of the partial pressure of ethylene in fungus-inoculated or 2,4,5-T-treated carrot root discs. The results indicate that ethylene is required for the induction of MMHD formation by carrot root tissue.