Search

Eleven soybean cultivars with different levels of susceptibility to virulent isolate SG915 ofCalonectria ilicicola were examined for reaction to metabolites produced by the isolate. When the culture filtrate from isolate SG915 was applied to trifoliates from 11 cultivars, cvs. ‘Cajun’ and ‘Asgrow 7986’ exhibited reduced wilting severity. However, there was no correlation between sensitivity to culture filtrate and susceptibility to the fungal isolate. Wilting severity on cv. ‘Riverside 699’ was greatest when trifoliates were treated with culture filtrates from isolates SG915 (highly virulent) and C31 (less virulent). The dilution end-point for culture filtrates of virulent isolate SG915 was determined to be 1:8. Nonautoclaved culture filtrates caused complete wilt of soybean trifoliates after 36 h, but autoclaved culture filtrates demonstrated a reduced ability to wilt leaves. Electrolyte leakage from treated leaf tissues increased over time regardless of the concentrations of culture filtrate tested. The greatest electrolyte losses were observed during the initial 30 min incubation of leaf tissues. The highest concentration of culture filtrate (50%, v/v) induced more electrolyte loss than the low concentration (10%, v/v) or control. These results suggest that toxic metabolites ofC. ilicicola may be involved in disease development with leaf symptom expression.

Greenhouse studies were conducted to determine host status of weed species for Rhizoctonia solani AG-I, which causes Rhizoctonia foliar blight of soybean. Weed species were barnyardgrass, broadleaf signalgrass, common cocklebur, entireleaf morningglory, hemp sesbania, itchgrass, johnsongrass, large crabgrass, northern jointvetch, prickly sida, purple nutsedge, redweed, sicklepod, and smooth pigweed. Seedling weeds were inoculated with suspensions containing intraspecific group IA and IB isolates of the fungus. In the first study, sclerotia of IA were recovered from tissue of all weeds except smooth pigweed, and mycelia of IA were recovered from all except smooth pigweed and redweed. In that study, neither microsclerotia nor mycelia of IB were recovered from sicklepod, barnyardgrass, or large crabgrass, and only microsclerotia were recovered from itchgrass and purple nutsedge. In the second study, sclerotia of IA, microsclerotia of IB, and mycelia of each isolate were recovered from all weed species. In other studies, R. solani spread from at least six of seven weed species to a noninfected soybean plant growing in close proximity. These studies emphasize the importance of weed control, not only for reducing plant competition and increasing yield, but also for the potential impact on development of RFB. Nomenclature: Rhizoctonia solani Kuhn AG-I IA; Rhizoctonia solani Kuhn AG-1 IB; barnyardgrass, Echinochloa crus-galli (L.) Beauv. # 3 ECHCG; broadleaf signalgrass, Brachiaria platyphylla (Griseb.) Nash # BRAPP; common cocklebur, Xanthium strumarium L. # XANST; entireleaf morningglory, Ipomoea hederacea var. integriuscula Gray # IPOHG; hemp sesbania, Sesbania exaltata (Raf.) Rybd. ex A. W. Hill # SEBEX; itchgrass, Rottboellia cochinchinensis (Lour.) W. Clayton # ROOEX; johnsongrass, Sorghum halepense L. Pers. # SORHA; large crabgrass, Digitaria sanguinalis (L.) Scop. # DIGSA; northern jointvetch, Aeschynomene virginica (L.) B. S. P. # AESVI; prickly sida, Sida spinosa L. # SIDSP; purple nutsedge, Cyperus rotundus L. # CYPRO; redweed, Melochia corchorifolia L. # MEOCO; sicklepod, Senna obtusifolia L. # CASOB; and smooth pigweed, Amaranthus hybridus L. # AMACH; soybean, Glycine max (L.) Merr.

Isolates representing 11 anastomosis groups (AGs) of Rhizoctonia solani from various geographic locations and host plants were tested for virulence on soybean leaves at 15, 20, 25, 30, and 35°C, and on soybean seedlings at 20, 25, and 30°C. Numbers of infection cushions formed on soybean leaves were determined using light microscopy. Isolates of AG-1 IA, AG-1 IB and AG-5 were more virulent on soybean leaves at 20, 25, and 30°C than isolates of AG-1 IC and AG-4. Maximum numbers of infection cushions were formed on soybean leaves by AG-1 (IA, IB, and IC), AG-4, and AG-5 at 25 and 30°C. The other AGs tested did not form infection cushions on soybean leaves although some caused minimal disease severity. Isolates of AG-1 IA formed significantly more infection cushions and caused greater disease severity than AG-1 IB and other isolates at 35°C. Maximum seedling infection, based on per cent area of hypocotyl region covered by lesions occurred at 25 C for AG-1 (IA, IB, and IC) and AG-4. Isolates of AG-5 caused greater seedling infection at 20°C than at 25 and 30°C. The other AGs tested caused only minimal damage to the seedlings. Isolates of AG-4 and AG-5 are not known to cause Rhizoctonia foliar blights of soybean in Louisiana, but their potential to be destructive foliar pathogens is confirmed.

Infection cushions were formed by isolates of Rhizoctonia solani, anastomosis group 1 IA (AG-1 IA, aerial blight) and AG-1 IB (web blight) on leaves of all 10 soybean cultivars tested. Isolates of AG-1 IA and IB did not form infection cushions on soybean leaf surface replicas of either resistant or susceptible cultivars. More infection cushions were formed by isolates of AG-1 IA and IB on collodion membranes placed over leaves of susceptible cultivars compared with resistant cultivars. Isolates of AG-1 IC. AG-4 and AG-5, also formed infection cushions on soybean leaves. However, the isolates of other anastomosis groups did not form infection cushions on soybean leaves. Differential induction of infection cushion formation by the leaves of various plant species was observed, AG-1 IA formed infection cushions on more graminaceous hosts than AG-1 IB, Our results suggest that a chemical stimulus is needed for infection cushion formation. Glucose and 3-O-methylglucose repressed disease severity caused by AG-1 IA and IB isolates to the same extent. Disease severity and the number of infection cushions formed on leaves of ten soybean cultivars were correlated. Fewer infection cushions were formed on resistant cultivars than on susceptible cultivars.

Field experiments in 1988–1990 determined the (1) effects of defoliation by Pseudoplusia includens (Walker) (soybean looper) or basal stem girdling by Spissistilus festinus (Say) (threecornered alfalfa hopper) on soybean stem canker severity and (2) effects of the soybean looper defoliation on red crown rot incidence. Compared to ungirdled plants, stem canker severity was increased while stem length, stem diameter, and yield were reduced for girdled plants. Stem canker severity, number of perithecia, yield, stem length, and stem diameter were reduced on defoliated soybean, compared to undefoliated soybean. Red crown rot incidence, perithecial production, and yields were reduced on defoliated soybean compared to undefoliated soybean. This suggests different types of insect injury may either increase or decrease the severity of soybean diseases.

Short-term greenhouse studies with soybean (Glycine max cv. Bragg) were used to examine interactions between the soybean cyst nematode (Heterodera glycines) and two other common pests of soybean, the stem canker fungus (Diaporthe phaseolorum var. caulivora) and the soybean looper (Pseudoplusia includens), a lepidopterous defoliator. Numbers of cyst nematode juveniles in roots and numbers of cysts in soil and roots were reduced on plants with stem cankers. Defoliation by soybean looper larvae had the opposite effect; defoliation levels of 22 and 64% caused stepwise increases in numbers of juveniles and cysts in both roots and soil, whereas numbers of females in roots decreased. In two experiments, stem canker length was reduced 40 and 45% when root systems were colonized by the soybean cyst nematode. The absence of significant interactions among these pests indicates that the effects of soybean cyst nematode, stem canker, and soybean looper on plant growth and each other primarily were additive.

Greenhouse studies were done to determine effects of soybean looper, Pseudoplusia Includes (Walker), stem canker fungus, Diaporthe phaseolorum (Cke. and E11.) Sacc.var. caulivora Athow and Caldwell, and soybean cyst nematode, Heterodera glycines Ichinohe (race 3), on growth, nodule development, and N2-fixing ability (measured by acetylene-ethylene assay) of ‘Bragg’ soybean, Glycine max (L.) Merrill. Soybean looper larvae and stem canker fungus reduced soybean growth, nodule development, and N.-fixing ability. Reductions in N2-fixing ability caused by insect-induced defoliation resulted from decreases in number and weight of nodules but not from decreased nodule efficiency (nodule-specific activity), whereas reductions in this parameter in plants with stem cankers resulted from decreases in number, weight, and efficiency of nodules on roots. Soybean cyst nematode (race 3) had little measurable effect on most parameters measured, Effects of insect, fungus, and nematode primarily were additive.

Greenhouse and outdoor studies evaluated weed species common to Louisiana soybean (Glycine max) fields as potential hosts for Diaporrhe phaseolorum var. caulivora. Species tested were: barnyardgrass (Echinochloa crus-galli), black nightshade (Solanum nigrum), curly dock (Rumex crispus), entireleaf morning-glory (Ipomoea hederacea var. integriuscula), hairy indigo (Indigofera hirsuta), hemp sesbania (Sesbania exaltata), ivy-leaf morning-glory (Ipomoea hederacea), johnsongrass (Sorghum halepense), northern joint-vetch (Aeschynonsene virginica), pitted morning-glory (Ipomoea lacunosa), prickly sida (Sida spinosa), redweed (Melochia corchorifolia), sicklepod (Cassia obtusifolia), smallflower morning-glory (Jacquemontia tamnifolia), spiny amaranth (Amaranthus spinosus), tall morning-glory (Ipomoea purpurea), and wild poinsettia (Euphorbia heterophylla). Soybean cultivars susceptible to stem canker were included as controls. Plants were inoculated twice in each experiment using ascospores in water. Host status of each species was confirmed by recovering the pathogen from weeds using selective medium or by observing production of perithecia. All weed species tested except johnsongrass, barnyardgrass, and curly dock were hosts for D. p. var. caulivora. Lesions 1 to 2 cm long and ≤0.5 cm wide were common on soybean, hemp sesbania, and hairy indigo. These consistently yielded mycelium of D. p. var. caulivora. All other weed hosts were asymptomatic.

Double-stranded RNA (dsRNA) was detected in 22 of 35 (63%) Rhizoctoria safari anastomosis group one (AG-1) intraspecific groups 1A and 1B isolates screened. Most AG-1 IA isolates had seven dsRNAs with molecular sizes ranging from 1.3 to 9.3 kb, whereas most AG-1 IB isolates had a 12-kb dsRNA. Not all isolates of AG-1 IA or IB obtained from the same soybean field had dsRNA. The dsRNAs were stable through six successive subculturings. Cell fractionation of R. solani revealed that dsRNAs were located in the microsomal fraction. The dsRNAs were no longer detected in two isolates of AG-1 IB and one isolate of AG-1 IA after 1 wk of growth at 35 C; however, the dsRNAs of one isolate of AG-1 IA were not lost at 35 C [...]

A model to estimate wheat leaf rust severity at different positions from the tiller/whole plant rust severity values and disease progress curves was developed. Development of leaf rust on the ith leaf was expressed as dx it /dt=rX t (1-x it ) or, in an integrated form, x it =1-exp[-rX t (t-t i )], where x it is the rust severity on the ith leaf at time t, r is the tiller/plant apparent infection rate, X t is the tiller/plant rust severity observed at time t, and t i is the time of the ith leaf emergence (...)

The formation of perithecia and pycnidia and the release of ascospores and conidia, respectively, by Diaporthe phaseolorum var. caulivora was determined on soil at matric potentials (ψ m ) between 0 and -0.32 MPa. D. p. caulivora infected internodes of susceptible soybean cv. Bedford were incubated at 24 ± 1 C on the surface of soil samples adjusted to ψm = 0.0, -0.01, -0.02, -0.04, -0.08, -0.16, or -0.32 MPa. The number of perithecia and pycnidia formed and the number of sporulated perithecia and pycnidia that released ascospores and conidia, respectively, per square centimeter of soybean internode were counted at weekly intervals (...)

Incidence and severity of stem canker caused by Diaporthe phaseolorum var. caulivora were quantified from an inoculum point source in irrigated and nonirrigated plots of resistant, moderately resistant, and susceptible soybean cultivars (Bay, Wilstar 550, and Bedford, respectively) during the 1988-90 seasons. Steam canker severity at each disease assessment was higher on all cultivars in irrigated plots (...)

Each of six soybean fields with a history of Rhizoctonia foliar blight was divided into 0.75-×0.75-m quadrats. Diseased leaves in each quadrat were initially counted at soybean growth stage V8-V11. At soybean growth stage R4, disease incidence in each quadrat was assessed. Lloyd's index of patchiness (LIP) of diseased leaves ranged from 1.56 to 4.54 at the first rating. At the second disease rating, LIP of disease incidence for each plot ranged from 1.32 to 2.15, indicating a decrease in disease aggregation (except for 1987 Lake Arthur). Cluster size was examined using paired-quadrat variance analysis (...)

Surfactant- and nonsurfactant-containing commercial herbicide formulations of glyphosate were evaluated for in vitro and in vivo effects on Calonectria crotalariae and red crown rot of soybean, respectively. Rates of 0.28, 0.56, 1.12, and 2.25 kg of glyphosate per hectare, corresponding to recommended rates for weed control in soybean (Glycine max), were used. Three pathogenic isolates of the fungus from soybean were grown on a selective medium amended with either water or the different rates of the herbicide. Both formulations of glyphosate inhibited mycelial growth by C. crotalariae (...)

Four isolates of Colletotrichum capsici from pepper and cotton from Louisiana, and four isolates of C indicum from cotton from El Salvador, Thailand and India were studied for pathogenicity on unwounded cotton bolls and for morphological characters on the host and in culture which might serve to separate the two species. All isolates, regardless of host or geographic origin, were pathogenic on unwounded cotton bolls. Relative amount and location of fungal stromatic tissue development within boll tissues varied similarly for both species. Conidial dimensions were nearly identical for C. capsici and C. indicum. Colletotrichum indicum should be considered a synonym of C. capsici based on similar pathogenic and morphological characters.

(...) High numbers of primary infections were found early in the season. The correlation between average number of primary infections per day and average daily rainfall was highly significant. (...) The epidemiology of aerial blight may be divided into two phases, one before and one after canopy closure. The first phase is soilborne and determines the number of potential disease foci in the crop canopy. The second phase is leafborne and is important to the expansion of disease foci

(...) The average apparent infection rates of the epidemics were significantly different for different dates of inoculation. (...) Leaf rust severity was highly correlated with cumulative degree days following inoculation. (...) The logistic model adequately explained the development of leaf rust in time and space. Cultivar resistance affected leaf rust severity and extent of spread. (...) Yield reductions in all the cultivars were significantly related to areas under the disease progress curve and distance from focus. The significance of distance of spread in estimating yield reduction is discussed

Incidence and severity of stem canker arising from inoculum point sources of Diaporthe phaseolorum var. caulivora was quantified on resistant (Bay), intermediate (DPL105, W550) and susceptible (Bedford) soybean cultivars in the field during 1986 and 1987. Disease progress was described by the Weibull model. The shape parameter (c) of the model was equal to the idealized c value of a monocyclic disease for Bedford (...)

(...) Web blight caused by R. solani anastomosis group 1, intraspecific group IB (AG-1 IB), was the major type in Louisiana in both years. Aerial blight caused by isolates of R.s. AG-1 IA, previously considered the only type in the state, was also found. (...) Isolates of AG-1 IA and AG-1 IB could be differentiated on the basis of cultural characteristics on water agar and potato-dextrose agar. Maximum porduction of microsclerotia in the field occurred from late July through August. low numbers of microsclerotia produced by R.s. AG-1 IB were observed on plants at earlier and later soybean growth stages

(...) The isolate of R.s. had little effect on the number of seedlings emerging and surviving. Diseased seedlings became sources of inoculum by producing mycelium that grew to and infected neighboring plants. Seedling infection at an early stage thus can have a significant effect on subsequent disease development

(...) Simple linear regressions of the disease variables on days after inoculation showed increases in slopes as free moisture increased. Plant growth stage at inoculation also significantly affected the slopes. (...) The radius of a disease focus was a linear function of accumulated free moisture hours. Diseased plants/focus and disease incidence were best fitted with equation Y=3.412(Bt) 2 , where t is time and B are slopes and were proportional to plant growth stage. Correlation coefficients among disease incidence, focus radius, and diseased plants/focus ranged from 0.885 to 0.96 (...)

The effects of the combinations of leaf rust inoculation at different growth stages and initial inoculum levels on leaf rust development and yield of winter wheat cultivars, McNair 1003 and Coker 762 were evaluated. Disease onset stage and initial inoculum level affected the rate of leaf rust development and shape of the disease progress curves in both cultivars. Epidemics with common onset stages and different initial inoculum levels differed in area under the disease progress curve (AUDPC). Leaf rust epidemics initiated at Feeke's growth stages 5, 7, and 10 reduced yield in both cultivars. Leaf rust epidemics initiated early with high inoculum levels had the greatest deleterious effect on yield. Maximum losses due to leaf rust were 30–40 %. Yield loss was directly related to AUDPC when the AUDPC varied from 500 to 1700 in McNair 1003 and from 250 to 1700 in Coker 762. Yield reduction was mainly due to reduction in grain weight.