Plant Disease Severity Research Articles

Response of susceptible, partially resistant and resistant winter wheat cultivars to Blumeria graminis f.sp. tritici – short communication

Response of the susceptible cultivar Kanzler, the partially resistant cultivar Mikon and the resistant cultivar Asta (genes of resistance Pm2, Pm6) to powdery, were tested in two years small plot-experiments. Disease severity was influenced by weather conditions. There were highly significant differences in disease severity, infection type and number of diseased plants between the susceptible cultivar and the cultivars with partial resistance and specific resistance. Smaller differences were between the partially resistant cultivar and the resistant cultivar than between the cultivar with partial resistance and the susceptible cultivar. The most affected leaf was the third leaf from the top in all tested cultivars. Among these cultivars were differences in the highest development of disease in single growth stages.

Comparison of Plastic Mulch and Nitrogen Form on the Incidence of Verticillium Wilt of Eggplant.

Eggplants (cv. Midnite) were grown during 1996 to 1998 in soils naturally infested with Verti-cillium dahliae. Experimental plots were either mulched with black plastic or grown on bare ground and fertilized with either (NH4)2SO4 or Ca(NO3)2 at 224 kg of N per hectare each season. Compared to bare ground, mulch resulted in a 42 and 68% increase in 1996 and 1997, respectively. Compared to Ca(NO3)2, (NH4)2SO4 fertilization increased yield by 22 and 18% in 1996 and 1997, respectively. In both years, there were no significant interactions between the mulch and fertilizer treatments on yield or on integrated estimates of the plant canopy growth curve or disease severity. However, the treatment effects were additive. When compared to bare ground, mulching increased the plant canopy 3 weeks after planting and reduced the percentage of symptomatic foliage 8 weeks after planting. Compared to Ca(NO3)2, fertilization with (NH4)2SO4 increased the plant canopy after 6 weeks, but did not affect the percentage of symptomatic foliage. Mulching or (NH4)2SO4 fertilization increased the number of flowers compared to no mulching or Ca(NO3)2 fertilization, respectively. The rate of nitrogen fertilizer at planting or as a side-dress application did not affect growth or disease, but plants treated with (NH4)2SO4 were larger and had less symptoms than Ca(NO3)2-treated plants. The use of black plastic mulch with (NH4)2SO4 fertilization complements one another and may reduce damage from Verticillium wilt on eggplant.

Benzo-(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester induces systemic resistance in tomato ( Lycopersicon esculentum. Mill cv. Vollendung) to Cucumber mosaic virus

The potential of benzo-(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester (BTH) to trigger systemic acquired resistance (SAR) in tomato (Lycopersicon esculentum. Mill cv. Vollendung) plants against a yellow strain of Cucumber mosaic virus (CMV-Y) was investigated. Application of BTH, as a drench, 7 days before inoculation with CMV-Y, protected plants against the necrosis caused by CMV-Y. The resistance was evident as a decreased disease incidence and severity in BTH-treated plants. Twenty-one days after challenge inoculation with CMV-Y, the disease incidence in plants with SAR did not exceed 12.5% whereas, 91.7% of control plants were severely infected. The development of primary disease symptoms in BTH-treated plants was delayed for 7 days. The disease spread rapidly in control plants and by the end of the experiment almost all control plants showed severe mosaic and leaf necrosis. Results of enzyme-linked immunosorbent assay (ELISA) indicated that BTH treatment affected virus replication in protected leaf tissues. Analysis of the newly developed leaves of BTH-treated plants for virus antigen revealed that symptomless plants failed to support the replication of CMV-Y and the concentration of the virus in these plants was similar to that in uninoculated control plants.

Proportional odds model applied to mapping of disease resistance genes in plants

Molecular markers have been used extensively to map quantitative trait loci (QTL) controlling disease resistance in plants. Mapping is usually done by establishing a statistical association between molecular marker genotypes and quantitative variations in disease resistance. However, most statistical approaches require a continuous distribution of the response variable, a requirement not always met since evaluation of disease resistance is often done using visual ratings based on an ordinal scale of disease severity. This paper discusses the application of the proportional odds model to the mapping of disease resistance genes in plants amenable to expression as ordinal data. The model was used to map two resistance QTL of maize to Puccinia sorghi. The microsatellite markers bngl166 and bngl669, located on chromosomes 2 and 8, respectively, were used to genotype F2 individuals from a segregating population. Genotypes at each marker locus were then compared by assessing disease severity in F3 plants derived from the selfing of each genotyped F2 plant based on an ordinal scale severity. The residual deviance and the chi-square score statistic indicated a good fit of the model to the data and the odds had a constant proportionality at each threshold. Single-marker analyses detected significant differences among marker genotypes at both marker loci, indicating that these markers were linked to disease resistance QTL. The inclusion of the interaction term after single-marker analysis provided strong evidence of an epistatic interaction between the two QTL. These results indicate that the proportional odds model can be used as an alternative to traditional methods in cases where the response variable consists of an ordinal scale, thus eliminating the problems of heterocedasticity, non-linearity, and the non-normality of residuals often associated with this type of data.

Effect of crop rotation on the severity of grey leaf spot on maize

Grey leaf spot (GLS) caused by Cercospora zeae-maydis Tehon and Daniels has become the most important foliar disease of maize in South Africa. Crop rotation has been applied successfully to control or reduce the incidence and severity of plant diseases in general. GLS severity in maize monoculture and maize rotated with soybeans was monitored over four growing seasons. To reduce interplot effects experimental plots were surrounded by buffer rows of a resistant maize cultivar which was sprayed with fungicides every two weeks. Percentage GLS severity was visually assessed at fortnightly intervals. No significant differences in grey leaf spot severity were found between maize grown in monoculture or when rotated with soybeans. This result may be attributed to the ability of grey leaf spot spores to survive for two years in maize debris on the soil surface. It appears, therefore, that short term crop rotation will not control the disease under epidemic conditions.

Towards the Development of a Novel In Vitro Strategy for Early Screening of Fusarium Ear Blight Resistance in Adult Winter Wheat Plants

A novel in vitro bioassay is described for screening Fusarium ear blight (FEB) resistance in adult winter wheat plants. Seven winter wheat cultivars were assessed for components of partial disease resistance as 28 day-old detached leaf segments in the laboratory using isolates of Microdochium nivale var. nivale and M. nivale var. majus. Results were compared with disease data obtained at anthesis using the same cultivars as whole plants and the same isolates under glasshouse conditions. Significant cultivar differences were observed using detached leaves, with cv. Avalon (a Fusarium culmorum ear susceptible cultivar) having the shortest leaf incubation period, greatest leaf lesion development and shortest leaf latent period compared to cv. Spark (a Fusarium culmorum ear resistant cultivar), which had the longest leaf incubation period, least leaf lesion development and longest leaf latent period. Using whole plants, cv. Avalon had the shortest ear incubation period and greatest ear disease severity, whilst cv. Spark had the longest incubation period and least ear disease severity. Overall, cultivars of intermediate F. culmorum ear resistance expressed intermediate responses to M. nivale isolates, using both detached leaves and whole plants. Significant correlations were found with ear disease severity and ear incubation period in whole plants and components of partial disease resistance in detached leaves, with significant correlations obtained between leaf incubation period and ear disease parameters using the M. nivale var. nivale isolate. In addition, leaf latent period and leaf lesion size showed significant correlations with whole plant reactions using M. nivale var. nivale and var. majus isolates. The in vitro screening of cultivars as detached leaves using M. nivale isolates may offer a real possibility of a rapid bioassay for the early screening of FEB resistance in wheat and other cereals.

Effects of Initial Epidemic Conditions, Sporulation Rate, and Spore Dispersal Gradient on the Spatio-Temporal Dynamics of Plant Disease Epidemics

ABSTRACT A stochastic model that simulates the spread of disease over space and time was developed to study the effects of initial epidemic conditions (number of initial inocula and their spatial pattern), sporulation rate, and spore dispersal gradient on the spatio-temporal dynamics of plant disease epidemics. The spatial spread of disease was simulated using a half-Cauchy distribution with median dispersal distance mu (units of distance). The rate of temporal increase in disease incidence (beta(I), per day) was influenced jointly by mu and by the sporulation rate lambda (spores per lesion per day). The relationship between beta(I) and mu was nonlinear: the increase in beta(I) with increasing mu was greatest when mu was small (i.e., when the dispersal gradient was steep). The rate of temporal increase in disease severity of diseased plants (beta(S)) was affected mainly by lambda: beta(S) increased directly with increasing lambda. Intraclass correlation (kappa(t)), the correlation of disease status of plants within quadrats, increased initially with disease incidence, reached a peak, and then declined as disease incidence approached 1.0. This relationship was well described by a power-law model that is consistent with the binary form of the variance power law. The amplitude of the model relating kappa(t) to disease incidence was affected mainly by mu: kappa(t) decreased with increasing mu. The shape of the curve was affected mainly by initial conditions, especially the spatial pattern of the initial inocula. Generally, the relationship of spatial autocorrelation (rho(t,k)), the correlation of disease status of plants at various distances apart, to disease incidence and distance was well described by a four-parameter power-law model. rho(t,k) increased with disease incidence to a maximum and then declined at higher values of disease incidence, in agreement with a power-law relationship. The amplitude of rho(t,k) was determined mainly by initial conditions and by mu: rho(t,k) decreased with increasing mu and was lower for regular patterns of initial inocula. The shape of the rho(t,k) curve was affected mainly by initial conditions, especially the spatial pattern of the initial inocula. At any level of disease incidence, autocorrelation declined exponentially with spatial lag; the degree of this decline was determined mainly by mu: it was steeper with decreasing mu.

A Leaf Inoculation Method for Detection of Xanthomonas oryzae pv. oryzicola from Rice Seed.

A leaf-inoculation method for detecting Xanthomonas oryzae pv. oryzicola in rice seed was developed and tested. The method is based on inoculating leaf segments on agar with seed washings in a moist chamber. Freshness of inoculated leaf segments is maintained by addition of 75 to 100 ppm benzimidazole to the 1% water agar after sterilization. Using cells from pure cultures, the minimum inoculum concentration to initiate leaf streak lesions on the segments was found to be 103 CFU/ml. Inoculum prepared from seeds harvested from severely infected mother plants induced lesions typical of leaf streak, followed by bacterial ooze. With 4 g of seed, the incubation period was 3.5 days and bacterial ooze appeared within 5 days. The minimum amount of seed needed to detect the bacterium based on seed washings varied according to disease severity of the mother plants. The bacteria isolated from lesions on the leaf segments were confirmed as X. oryzae pv. oryzicola after bacteriological, serological, and Biolog tests were conducted. The results confirm that the detached-leaf method is as simple and reliable as the immuno-radiometric assay and inoculation on intact pot-grown rice plants in the greenhouse.

Abstracts, Saskatchewan Regional Meeting, Canadian Phytopathological Society, Saskatoon, Saskatchewan, 20 March 1997

Yield losses from diseases on cereals and canola grown in Saskatchewan. K.L. Bailey, L. Boyko, A.J. Johnston, and R. A. A. Morrall. Agriculture and Agri-Food Canada, 107 Science Place, Saskatoon, Saskatchewan S7N 0X2; (AJ.J.) Agriculture and Agri-Food Canada. P.O. Box 1240, Melfort, Saskatchewan SOE IA0; (R.A.A.M.) Department of Biology, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2. Sclerotinia stem rot resistance in canola petals. C. Lefol, G. Séguin-Swartz, and R.A.A. Morrall. Department of Biology, University of Saskatchewan, 112 Science Place, Saskatoon, Saskatchewan S7N 5E2; (G.S.S.) Agriculture and Agri-Food Canada Research Centre, 107 Science Place, Saskatoon, SK, S7N0X2. The use of a 6-glucuronidase-marked isolate of Phoma lingam to study the reaction of selected crucifers to the blackleg fungus. C.Y. Chen and G. Séguin-Swartz. Agriculture and Agri-Food Canada Research Centre, 107 Science Place, Saskatoon, Saskatchewan S7N 0X2. Incidence of Mycosphaerella pinodes in field pea seed samples, 1995-96. B.D. Gossen and S.V. Rude. Agriculture and Agri-Food Canada, Saskatoon Research Centre, 107 Science Place, Saskatoon, Saskatchewan S7N0X2; (S.V.R.) Discover)' Seed Labs Ltd., #4 - 1527 Ontario Ave., Saskatoon, Saskatchewan S7K IS7. Improvement in control of velvetleaf with Colletotrichum gloeosporioides f. sp. malvae. H.R. Kutcher. Saskatoon Research Centre, Agriculture and Agri-Food Canada, 107 Science Place, Saskatoon, Saskatchewan S7N0X2. Impact of different types and concentrations of Pythium ultimum inoculum on disease severity and biocontrol efficacy in greenhouse ornamental plants. M. Sulz, G. Peng, N. Seresinhe, and N. Nyssen. Agrium Inc., Suite 402, 15 Innovation Boulevard, Saskatoon, Saskatchewan S7N 2X8.

Comparison of disease measures for assessing resistance in canola (Brassica napus) to blackleg (Leptosphaeria maculans)

Disease incidence, disease severity, areas under the disease incidence and disease severity progress curves, and maximum and minimum incidence and severity of disease were evaluated as measures of resistance in canola (Brassica napus) to blackleg caused by Leptosphaeria maculans. Seven genotypes of spring canola were tested in Ontario at three locations 60, 81, and 102 days after sowing in 1990 and at two locations 55, 70, 85, and 100 days after sowing in 1991. Disease incidence was expressed as the percentage of sampled plants with basal stem canker, and disease severity was assessed as the percentage discolouration of a cross section of the stem base. Based on ease of measurement, range of values, number of significant differences among genotypes, and constancy of rank of genotypes at different locations, measures of disease severity or disease incidence in mature plants proved to be most useful. No advantage was gained by using area under the disease incidence and disease severity progress curves, or maximum and minimum incidence and severity of disease at the last sampling time. It is recommended that resistance of canola stems to blackleg be evaluated from measures of disease severity or disease incidence in mature plants. Keywords: canola, blackleg, Leptosphaeria maculans, resistance measurements.

Ultraviolet-Absorbing Plastic Sheets Protect Crops from Insect Pests and from Virus Diseases Vectored by Insects

Crops grown in walk-in tunnels covered with UV-absorbing plastic sheets were highly protected from infestation with sweetpotato whitefly, Bemisia tabaci (Gennadius); western flower thrips, Frankliniella occidentalis (Pergande); and cotton aphid, Aphis gossypii Glover. Protection against B. tabaci was associated with a dramatic reduction in the spread of the whitefly-borne geminivirus tomato yellow leaf curl (TYLCV) and with a pronounced delay in plant infection and disease severity. Two months after transplantation, the average TYLCV disease incidence under 3 different UV-absorbing plastic sheets was 50, 30, and 20% compared with 93% under UV-nonabsorbing polyethylene sheets. In additional experiments, it was shown that the UV-absorbing sheets were also highly effective in the protection of cucumbers against F. occidentalis and A. gossypii . Filtration of full spectrum and monochromatic UV light by UV-absorbing plastic sheets reduced significantly the attraction of B. tabaci to these light sources. The association of the described phenomena with the UV vision of insects is discussed.

Biological Control ofMonilinia laxaandFusarium oxysporumf. sp.lycopersiciby a Lytic Enzyme-ProducingPenicillium purpurogenum

Abstract Biological control experiments were conducted with the lytic enzyme-producing fungus Penicillium purpurogenum against the plant pathogens Monilinia laxa and Fusarium oxysporum f. sp. lycopersici. Applications of P. purpurogenum to peach shoots previously inoculated with M. laxa reduced lesion length and extent of pathogen colonization of shoots by 90 and 80% ( P ≤ 0.05), respectively, comparable to the level of disease control obtained with the fungicide captan. Disease severity in tomato plants inoculated with F. oxysporum f. sp. lycopersici was decreased by 30% ( P ≤ 0.05) with the biological treatment. The fungus P. purpurogenum produced β-1,3-glucanase and chitinase activities in liquid culture that were inducible by cell walls and live mycelium of M. laxa but not of F. oxysporum f. sp. lycopersici. Crude filtrates or crude enzyme preparations of P. purpurogenum cultures with lytic enzyme activities produced lysis of hyphae and spores of M. laxa and F. oxysporum f. sp. lycopersici. These lytic effects were strong in M. laxa and ended in complete dissolution of mycelium. The induction of lytic enzymes by M. laxa and the effects of lytic enzymes on mycelia of the pathogens in relation to the different degrees of biological control obtained are discussed.

Deficit Irrigation of Micro-Irrigated Tomato Affects Yield, Fruit Quality, and Disease Severity

Florida's water resources have been burdened by competition between a steadily increasing population and 2 million acres of irrigated agriculture. During chronic dry weather, regulatory agencies may impose mandatory cutbacks in water drawn for agricultural use to protect groundwater levels, regardless of irrigation methods used by growers. We investigated the impact of water application deficits on tomato (Lycopersicon esculentum Mill. 'Sunny') under micro-irrigation (drip irrigation), where normally little surplus water is applied above crop requirements. Full-bed plastic-mulched tomato was grown for three springs on a sandy Spodosol in south Florida. Following 3 wk for crop establishment, three irrigation treatments were applied through the rest of the growing season: (i) full irrigation based on daily evapotranspiration (ET) estimates and soil water tension, (ii) 15% deficit from full irrigation, and (iii) 30% deficit. Treatments were applied daily by adjusting the irrigation time. Estimated seasonal tomato plant ET ranged between 5.5 and 7.4 in., and full irrigation ranged between 3.9 and 7.1 in. Soil water tension at the 6-in. soil depth under the micro-irrigation tubing fluctuated between 2 and 20 cbars, and was consistently greater with decreased irrigation. Tomato plant height decreased about 1.5 in., and crop water stress index (CWSI) increased with decreased irrigation. In 1992, early blight disease (caused by the fungus Alternaria solari) incidence was 50% more severe, and in 1994, blossom end rot incidence was five times more severe in the 30% deficit treatment compared with full irrigation. Three-year mean total marketable tomato yield decreased from 11.09 to 8.83 tons/acre as irrigation decreased. The yield decrease was chiefly due to decreased production of extra-large fruit, which receive the highest price on the fresh market. An economic analysis showed that 15 and 30% irrigation reductions would reduce gross revenue by 15 and 22%, respectively. Therefore, deficit-irrigating micro-irrigated tomato could cause substantial economic loss through decreased crop marketability.

バラ葉外植体の再生個体におけるうどんこ病抵抗性系統の選抜

Rose plants moderately resistant to the powdery mildew were selected from leaf callusderived regenerants. The process was facilitated by developing an efficient system for inoculation with the powdery mildew pathogen, Sphaerotheca pannosa. Fifty regenerants obtained from leaf explants of rose (Rosa hybrida cv. Carl Red) were grown and propagated in a greenhouse. The compound leaves of the fourth node of the new shoots were used to inoculate with conidiospores of S. pannosa KS-pl, using an apparatus devised for a large scale inoculation. Seven days after inoculation, disease severity of test plants was determined by a scale ranging from asymptomatic (0) to entirely pustule-engulfed leaves (3). Control leaves prepared from susceptible 'Carl Red' and resistant 'Magic' were rated 2.8 and 0, respectively, for disease severity. Some regenerants fell into the range of resistance, in particular, RHCL-203 and -263, which were rated 1.1 and 1.2, respectively. When leaves were harvested at random from these two regenerants cultivated in a greenhouse and retested for their resistance to the pathogen, their disease severities were ranked as being moderately resistant to powdery mildew. Therefore, these regenerants were selected as possible candidates for commercial use.

Interval mapping of genes for quantitative resistance of maize to Setosphaeria turcica, cause of northern leaf blight, in a tropical environment

Quantitative trait loci (QTL) involved in the resistance of maize to Setosphaeria turcica, the causal agent of northern leaf blight, were located by interval mapping analysis of 121 F2:3 lines derived from a cross between Mo17 (moderately resistant) and B52 (susceptible). A linkage map spanning 112 RFLP loci with 15 cM mean interval length was constructed, based on marker data recorded in a previous study. Field tests with artificial inoculation were conducted at three sites in tropical mid- to high-altitude regions of Kenya, East Africa. Host-plant response was measured in terms of incubation period, disease severity (five scoring dates), and the area under the disease progress curve (AUDPC). Heritability of all traits was high (around 0.75). QTL associated with the incubation period were located on chromosomes 2S and 8L. For disease severity and AUDPC, significant QTL were detected in the putative centromeric region of chromosome 1 and on 2S, 3L, 5S, 6L, 7L, 8L and 9S. On 2S the same marker interval which carried a gene enhancing latent period was also associated with reduced disease severity of juvenile plants. QTL on chromosomes 3L, 5S, 7L and 8L were significant across environments but all other QTL were affected by a large genotype x environment interaction. Partially dominant gene action for resistance as well as for susceptibility was prevailing. Single QTL explained 10 to 38% of the phenotypic variation of the traits. All but the QTL on chromosomes 1, 6 and 9 were contributed by the resistant parent Mo17. On chromosome 8L a QTL mapped to the same region as the major race-specific gene Ht2, supporting the hypothesis that some qualitative and quantitative resistance genes may be allelic.

The Relationship Between Plant Disease Severity and Yield

The relationship between disease and yield is most often summarized as a simple empirical model that describes average crop performances is the presence of a pathogen. Such models may be robust and useful for surveys but their use is usually constrained to the specific conditions under which the modal was developed. Changes in production system usually invalidate the relationship. The alternative is to base the relationship on an epidemiological analysis of the pathogen population and a physiological concept of host growth and development. This review provides the knowledge and conceptual basis and discusses the limitations to progress in the development of such models. It is shown that a host-based assessment of disease is well suited to yield investigations and to multiple pest constraints, and that disease is logically related to yield via radiation interceptions and radiation use efficiency.

Effect of chloride fertilization on Bedford barley and Katepwa wheat

Recent studies in the northern Great Plains have confirmed that the chloride (Cl−) component of fertilizers can reduce disease severity and increase grain yield for wheat (Triticum aestivum) and barley (Hordeum vulgare). Field studies were conducted in Manitoba in 1989 and 1990 to determine the effect of rates of 25 and 50 kg Cl− ha−1 (applied as KCl or NaCl) applied with or without Cochliobolus sativus inoculum on plant nutrient status, disease severity and grain yield for Katepwa wheat and Bedford barley. Chloride application, regardless of placement or source, increased the Cl− concentration in plant tissue sampled at the boot to heading stages. Rates of 25 and 50 kg Cl− ha−1 resulted in significant reductions in the severity of common root rot for barley in two of six experiments and for wheat in one of four experiments. Chloride applications did not reduce spot blotch severity on barley in either of two experiments conducted. Inoculum did not have a consistent effect on any of the parameters measured. The application of 50 kg Cl− ha−1 significantly increased grain yield for barley by an average 393 kg ha−1 in two of eight experiments, but did not increase grain yield for wheat in any of eight experiments. Yield responses to Cl− were not related to soil Cl− content, Cl− concentration in plant tissue or observed reductions in disease. Key words: Chloride, fertilizers, barley, wheat, disease, yield

The Relationship between Plant Disease Severity and Yield

The Relationship between Plant Disease Severity and Yield

Plant Disease Severity in High-Input Compared to Reduced-Input and Organic Farm Systems

Plant Disease Severity in High-Input Compared to Reduced-Input and Organic Farm Systems

Periplasmic disulphide bond formation is essential for cellulase secretion by the plant pathogen Erwinia chrysanthemi

Secretion to the cell exterior of cellulase EGZ and of at least six pectinases enables the Gram-negative Erwinia chrysanthemi to cause severe plant disease. The C-terminal cellulose-binding domain (CBD) of EGZ was found to contain a disulphide bond which forms, in the periplasm, between residues Cys-325 and Cys-382. Dithiothreitol (DTT)-treatment of native EGZ showed that the disulphide bond was dispensable, both for catalysis and cellulose binding. Adding DTT to E. chrysanthemi cultures led to immediate arrest of secretion of EGZ which accumulated in the periplasm where the CBD was eventually proteolysed. Site-directed mutagenesis that affected Cys residues involved in disulphide bond formation resulted in molecules that were catalytically active and able to bind to cellulose but were no longer secreted. Instead they accumulated in the periplasm. Interestingly, the region around EGZ Cys-325 is conserved in two pectinases secreted by the same pathway as EGZ. We conclude that the conserved Cys, and possibly adjacent residues, bear essential information for EGZ to be secreted and that periplasmic disulphide bond formation is an obligatory step which provides a pre-folded functional form of EGZ with secretion competence.