Foliar Anthracnose Research Articles

Colletotrichum species associated with atemoya anthracnose in northeast Brazil

Anthracnose, caused by Colletotrichum spp., is an important disease of atemoya that can infect and cause damage to different plant organs. The symptoms include foliar anthracnose, flower abortion, branch tip bleaching, and necrotic lesions on fruits. Knowing the etiology of this disease is essential for properly developing management strategies. Therefore, this study aimed to identify Colletotrichum species associated with atemoya in the Northeast region of Brazil, based on multi-locus phylogeny and morphological analyses, to better understand the etiology of anthracnose in this plant species. The Colletotrichum isolates were obtained from symptomatic leaves and branches of atemoya collected in commercial fields in Vitória da Conquista (Bahia), Palmeira dos Índios and Rio Largo (Alagoas), located in northeast Brazil. The initial analysis of the partial sequences of the GAPDH gene was performed with nineteen Colletotrichum isolates obtained from leaves and branches of atemoya trees with typical anthracnose symptoms. Seventeen isolates were grouped in the gloeosporioides complex and two in the boninense complex. Therefore, eleven isolates were submitted to concatenated phylogenetic analyses with the genes GAPDH, TUB2, and ITS region of the rDNA to confirm the identity of the isolates at the species level. The species C. theobromicola, C. siamense, C. fructicola and C. karstii were identified and are associated with atemoya anthracnose in the Northeast region of Brazil.

Avaliação da sensibilidade micelial a fungicidas de Colletotrichum spp. associadas à atemoia no estado de Alagoas.

Anthracnose, caused by Colletotrichum spp., is an important disease of atemoya causing damage both pre and post-harvest. Symptoms of this disease range from foliar anthracnose, flower abortion, branch blight and necrotic fruit lesions. Knowing the action of certain fungicides in the fight against the etiological agent is essential to develop control techniques. The objective of this work was to evaluate the mycelial sensitivity of Colletotrichum spp., from atemoya, to the fungicides thiophanate methyl, azoxystrobin and tebuconazole. To determine the sensitivity of Colletotrichum species to fungicides, mycelial growth in synthetic potato-dextrose-agar (PDA) supplemented with fungicide was evaluated. The fungicides were dissolved in dimethylsulfoxide (DMSO) and added to the fluxing culture medium (45°C), to reach concentrations of 0.0, 0.1, 0.5, 1.0, 5.0 and 10 μg/ml-1 of a.i. The mycelial growth of each colony was measured daily to obtain the mycelial growth index (ICM). EC50 was also calculated. After calculating the EC50, the Colletotrichum species were classified into three sensitivity categories, where: EC50: <10 μg/ml-1: high sensitivity (SA); EC50: 10-100 μg/ml-1: moderate sensitivity (MS); EC50: 100-500 μg/ml-1: insensitivity (I). The species C. theobromicola, C. fruticola, C. siamense and C. karstii were highly sensitive to the fungicides tebuconazole and thiophanate methyl. The fungicide azoxystrobin proved to be efficient in the in vitro control of the species C. theobromicola, however, it proved to be inefficient for the control of the species C. siamense. The species C. fruticola and C. karstii were moderately sensitive to the fungicide azoxystrobin.

Diversity of Colletotrichum species causing onion anthracnose in Brazil

Foliar anthracnose is one of the main diseases of onion (Allium cepa L.) under tropical and subtropical conditions. Thus far, only Colletotrichum gloeosporioides has been reported as the causal agent of this disease in Brazil. However, there are no extensive studies characterizing Colletotrichum isolates associated with onion anthracnose in the country. Here, 38 Colletotrichum isolates obtained from onion plants displaying foliar anthracnose across major Brazilian onion-producing regions were characterized using morphometric and molecular information. The Bayesian and Maximum Liklihood methods were used for an initial analysis of the β-tubulin gene (tub2) sequences of all isolates, resulting in the discrimination of nine haplotypes. Three haplotypes grouped with the reference species of the C. acutatum complex and six with the C. gloeosporioides complex. Sequences of either the glyceraldehyde-3-phosphate dehydrogenase (gapdh), actin (act), and calmodulin genes or the intergenic spacer (IGS) region between DNA lyase (apn2) gene and the mating-type mat1–2-1 locus were used to characterize a subset of isolates representing these nine distinct tub2 gene haplotypes. These analyses revealed five anthracnose-inducing Colletotrichum species, including three members of the C. acutatum species complex (C. nymphaeae, C. scovillei, and C. tamarilloi) and two of the C. gloeosporioides species complex (C. fructicola and C. theobromicola). Bioassays confirmed that all these Colletotrichum species are pathogenic to onion, inducing typical anthracnose symptoms on bulbs and leaves. Twenty-six out of 38 isolates were identified as C. theobromicola, indicating this fungus as the prevalent anthracnose pathogen under Brazilian conditions. This Colletotrichum species diversity will affect anthracnose management strategies, including chemical and cultural control as well as the identification and deployment of onion cultivars with species-specific and/or wide-spectrum tolerance/resistance.

Effectiveness of Fungicides and Their Application Timing for the Management of Sorghum Foliar Anthracnose in the Mid-Atlantic United States.

Sorghum anthracnose (Colletotrichum sublineola) reduces grain yield up to 50% but suggested management tactics have not yet been developed for the mid-Atlantic United States, where warm, wet conditions favor disease. Under factorial arrangement, five fungicides plus a nontreated control and four application timings were compared for foliar anthracnose control, yield, and profitability of fungicide use in grain sorghum over eight site-years in Virginia and North Carolina. Anthracnose severity was rated at the hard dough stage, and grain yield was determined at harvest. Every percent increase in disease severity resulted in yield losses of 27 to 85 kg/ha. Pyraclostrobin and pyraclostrobin plus fluxapyroxad reduced anthracnose (P < 0.01), and three applications resulted in less disease and greater yield compared with single applications (P < 0.01). However, three applications exceed the labeled maximum application for the fungicides and are not economical. Among single applications, boot or flowering timings reduced disease, and flowering applications resulted in the overall greatest yield. Results suggest that when disease onset occurs at or prior to boot, a single application of pyraclostrobin-containing fungicide at or just prior to flowering reduces anthracnose, protects yield, and increases income. However, when disease is absent or severity is low prior to flowering, fungicide application may not be profitable.

Minimizing yield and quality losses in watermelon with multi-site and strobilurin fungicides effective against foliar and fruit anthracnose

Anthracnose, caused by Colletotrichum orbiculare race 2, has re-emerged as a problematic disease on watermelon in the southern United States where the majority of U. S. watermelons are produced. Although several new fungicide active ingredients have been registered on cucurbits, their effects on watermelon yields produced during anthracnose epidemics have not been assessed. Field experiments were done in 2014 and 2017 on seeded watermelon treated with 13 fungicides in FRAC Groups 1, 3, 7, 9, 11, 27, M03, and M05. Trifloxystrobin, azoxystrobin, pyraclostrobin, thiophanate-methyl, chlorothalonil, and mancozeb consistently reduced the severity of foliar anthracnose compared to water and to the other seven fungicides tested. In general, mancozeb, chlorothalonil, and trifloxystrobin increased the numbers and weights of total and marketable-sized fruit and reduced the percentage of fruit with symptoms of anthracnose compared to the water control and to FRAC Group 7 fungicides. These three fungicides also increased the soluble solids content of fruit in 2014. Multi-site fungicides and strobilurin fungicides were more effective against anthracnose than fungicides in FRAC Groups 3, 7, 9, and 27.

English

South American Leaf Blight (SALB) of the rubber tree, caused by Microcyclus ulei and foliar anthracnose caused by Colletotrichum gloeosporioides, are diseases that adversely affect rubber cultivation in America. Both diseases have a significant economic impact on this agricultural subsector. The aim of the present study was to evaluate the potential as biological control agents of three Streptomyces species strains, namely A20, 7.1 and 5.1, against M. ulei and C. gloeosporioides. The results of analysis of variance (ANOVA) and Tukey post-hoc test of the in vitro antifungal activity assays evidenced the potential of the three Streptomyces strains to inhibit C. gloeosporioides growth through the production of diffusible (A20 and 5.1) and volatile compounds (7.1). Furthermore, other results indicated that strain 5.1 had a high biocontrol activity against C. gloesporoides, and thus such strain was selected for further evaluations as a possible biocontrol agent against M. ulei. In vitro assays suggested that active compounds produced by 5.1 inhibited M. ulei growth by interfering with conidia germ tube and stroma formation. Bioassay-guided fractionation with organic solvents of 5.1 fermentation broths, suggested that antifungal compounds produced by this strain were nonionic compounds of medium-polarity. Currently, studies are ongoing to elucidate the chemical structure of these antifungal compounds. These approaches aim to generate a biological control agent to provide the Colombian rubber subsector with a preventive measure for controlling M. ulei and C. gloeosporioides. Key words: Biological control, foliar anthracnose, rubber tree, South American Leaf Blight (SALB), Streptomyces species.

Strawberry foliar anthracnose assessment by hyperspectral imaging

Hyperspectral imaging provides comprehensive spectral and spatial information about observed objects. This technology has been applied to many fields, such as geology, mining, surveillance and agriculture. Strawberry qualities have been examined using hyperspectral imaging in several studies. However, none of the previous literature presented a non-destructive method for diagnosing the infection stages of anthracnose, a devastating disease for strawberries. This study examined strawberry foliar anthracnose using three different hyperspectral imaging analyzing methods: spectral angle mapper (SAM), stepwise discriminant analysis (SDA) and self-developed correlation measure (CM). Three different infection stages, including healthy, incubation and symptomatic stages, were investigated using these methods. The incubation stage is a stage at which the symptoms are still not yet visible. The three infection stage classification results were promising, with a classification accuracy of approximately 80%. For two infection stage classification (healthy and symptomatic stages), an average accuracy of high 80% was attained. In fact, an average accuracy of 93% was achieved by SDA for two-stage classification. This study not only proves the feasibility of hyperspectral imaging for diagnosing strawberry foliar anthracnose infection, but also identifies a smaller set of significant wavelengths at which similar classification performance was accomplished. For significant wavelength selection, partial least squares (PLS) regression is an standard wavelength selection method and it was applied to be compared with SDA and CM. Wavelengths of 551, 706, 750 and 914nm formed the multispectral imaging observing bands that showed an accuracy of 80% when classifying the three infection stages. Therefore, using either hyperspectral or multispectral imaging to detect anthracnose infected foliar areas is more practical and efficient than classic destructive methods. In particular, early detection (the incubation stage), something that cannot be seen via naked eyes, reaches 80% classification accuracy with both SDA and CM. Strawberry farmers could profit greatly from this technology.

Genotype rotation for leaf anthracnose disease management in sorghum

The aim of this study was to evaluate the use of genotype rotation as a sustainable management strategy for the control of foliar anthracnose of sorghum. Three grain sorghum hybrids, one susceptible (BR 304) and two resistant to anthracnose (1G150 and DAS740) were tested in continuous planting and genotype rotation during seven growing seasons. The area under disease progress curve (AUDPC) of sorghum anthracnose severity was significantly lower in the genotype rotation than in the continuous planting. The disease severity in the susceptible genotype BR 304 in rotation with the resistant genotypes 1G150 or DAS740 decreased about 42% and 70% respectively, compared to its continuous planting. A significant reduction of leaf blight disease severity caused by Exserohilum turcicum in DAS740 planted rotated with BR 304 was also observed. The grain yields of the three genotypes were significantly higher in the rotation system compared to continuous planting. The genotype rotation can be considered a practical management strategy for reducing the impact of anthracnose and other diseases in sorghum.

Field Assessment of Anthracnose Disease Response for the Sorghum Germplasm Collection from the Mopti Region

Problem statement: Sorghum anthracnose (Colletotrichum sublineolum) is a highly variable pathogen and new sources of host plant resistance are required for the development of resistant sorghum varieties. Germplasm collections are an important source of host plant resistance and screening germplasm will be essential to identify new sources of resistance. Approach: The sorghum (Sorghum bicolor) collection from the Mopti region of Mali was inoculated with Colletotrichum sublineolum and evaluated for foliar anthracnose disease response in Isabela, Puerto Rico during the 2004 and 2005 growing seasons using a partially balanced lattice design with three replications. Results: A resistant response was observed for 45 of the 97 accessions in the collection and mean infection severity for the 52 susceptible accessions was 27.6%. An association was observed between resistance and the administrative district where the germplasm was collected. More than 50% of the accessions from the Bandiagara and Bankass districts showed a resistant response. The lowest frequency of resistant germplasm was observed for the Mopti district with 25% of the accessions showing a resistant response. The susceptible accessions from the Mopti district, however, showed the lowest mean infection severity. Approximately 44% of the accessions from the Douentza district showed a resistant response with the susceptible accessions showing the highest mean infection severity. These results suggest an association between annual rainfall and anthracnose resistance, with sorghum accessions from drier regions showing greater susceptibility. Anthracnose resistance also showed an association with sorghum race classification and race guinea accessions were more frequently resistant as compared to accessions classified as race durra or durra-bicolor. Conclusion: The results indicated that anthracnose resistant sorghum germplasm is frequent in the Mopti region of Mali and that ecogeographic origin and sorghum characterization information can be used to aid in germplasm selection or germplasm acquisition to identify anthracnose resistant sources.

Field Evaluation of Anthracnose Resistance for Sorghum Germplasm from the Sikasso Region of Mali

Sorghum anthracnose is a highly variable pathogen and occurs in most sorghum producing regions worldwide. The disease can be managed using resistant varieties, but additional sources of resistance are needed for sorghum improvement. Germplasm collections are an important resource for the identification of host plant resistance and the 132 sorghum landraces from the Sikasso region of Mali were inoculated with Colletotrichum sublineolum and evaluated for foliar anthracnose resistance in Isabela, Puerto Rico during the 2004 and 2005 growing seasons. A resistant response was observed for 109 accessions. For the 23 susceptible accessions, infection severity was low with a mean infected leaf area of 5.6%. Only one accession was rated as highly susceptible. Anthracnose resistant germplasm was more frequently associated with the southern administrative districts that experience higher annual rainfall. More than 80% of the accessions from the Bougouni and Sikasso districts showed a resistant response. The lowest mean infection severity, 1.5%, was observed for accessions from the Sikasso district. In comparison, 71% of the accessions from the drier, northern Koutiala district showed a resistant response, with a mean infection severity of 8.8% for the susceptible accessions from this district. Nearly all landraces from the Sikasso region were classified as race guinea and the guinea landraces that were further classified as margaritiferum were all rated as resistant. Results indicate that the Sikasso region of Mali could be an important source of anthracnose resistant germplasm and that ecogeographic origin and race classification could be used to select germplasm for additional disease evaluations.

Plant Growth–Promoting Pseudomonas sp. Strains Reduce Natural Occurrence of Anthracnose in Soybean (Glycine max L.) in Central Himalayan Region

Biological control is an accepted important component of current plant disease management strategies. Introduction of bacterized seeds carrying bacterial isolates with proven growth-promotion capabilities and antagonistic characteristics offer a valid alternative to chemical protectants. Root colonization of disease-susceptible (PS 1024) and moderately resistant (PS1042) varieties of soyabean (Glycine Max L) by fluorescent pseudomonad (FLPs) strains GRP3, PEn-4, PRS1, and WRS-24 was studied in relation to natural occurrence of anthracnose caused by Colletotrichum dematium (Pers Ex Fr.) Grove. Rhizoplane population of FLPs was maintained at a critical level (5.3 cfu) up to 30 days of plant growth, followed by a steep decline. Indigenous FLPs population, however, remained nearly unchanged (3.0 to 2.4 log g(-1) root) between 30 days and 75 days of plant growth. The relative FLPs population in rhizosphere was lower than that in rhizoplane. Although intervarietal difference was observed, the root/shoot length remained unaffected. Compared to nonbacterized control, dry root weight was improved by FLPs treatment. Severity of foliar anthracnose was reduced significantly after FLPs treatment in the variety PS 1042. Because the point of FLPs treatment (seed bacterization) was away from the site of disease appearance (leaf), operation of induced systemic resistance in strains PEn-4 and GRP3 appears imminent.

English

Water yam (Dioscorea alata L.) is the most widely cultivated yam species globally. The major limitation to the profitable and sustainable production of D. alata is its susceptibility to anthracnose disease. The availability of resistant varieties could potentially form the cornerstone of an integrated management strategy for yam anthracnose; however, anthracnose resistance breeding is hampered by the dearth of knowledge on pathogen identity and diversity. Four forms of Colletotrichum are now known to be associated with foliar anthracnose of yam: the slow-growing grey (SGG), the fast-growing salmon (FGS), the fast-growing olive (FGO), and the fast-growing grey (FGG) forms. The close phylogenetic relationship of the first three forms to reference isolates of Colletotrichum gloeosporioides, and the fact that only strains of these forms have been observed to induce typical anthracnose symptoms on D. alata, recently confirmed that C. gloeosporioides is the causal agent of yam anthracnose disease. The FGG form possibly represents a distinct, endophytic, species as indicated by morphological, biological and molecular criteria. Previous research emphasized epidemiology and control but limited progress was made in understanding yam anthracnose disease based on this classical approach. Molecular approaches have started to unravel the systematics and ecology of Colletotrichum strains associated with yam anthracnose, as well the population biology of C. gloeosporioides on yam. Sexual recombination is a likely mechanism contributing to the high genetic diversity of C. gloeosporioides in yam-based cropping systems. Studies have been initiated to understand the mechanisms that generate genetic variation in C. gloeosporioides, and to gain some insight into the biochemistry of the interactions between the pathogen and yam. Our thesis in this article is that integrating traditional and molecular approaches to understanding C. gloeosporioides systematics, epidemiology and population genetics will lead to a much better understanding of yam anthracnose disease, and thus to the development of effective and sustainable control measures. Research successes and challenges are discussed, as well as their implications for future studies on pathogen evolutionary potential, anthracnose resistance breeding, and the deployment of resistance genes. Key words : Anthracnose, Colletotrichum gloeosporioides, Dioscorea spp., molecular markers, molecular systematics, population biology, resistance breeding, yam. African Journal of Biotechnology Vol. 2 (12), pp. 486-496, December 2003

A antracnose do sorgo

A antracnose, causada pelo fungo Colletotrichum graminicola, é a mais importante doença da cultura do sorgo (Sorghum bicolor) no Brasil. São reconhecidas três fases da doença: a antracnose foliar, a fase de podridão do colmo e a antracnose da panícula e dos grãos, sendo a fase foliar, a mais destrutiva, normalmente observada a partir de 30 a 40 dias após a emergência no estádio de desenvolvimento 4,0. O fungo Colletotrichum graminicola pode sobreviver por até 18 meses na ausência do hospedeiro, como micélio e conídios em restos culturais na superfície do solo, em hospedeiros alternativos e ainda como micélio, conídios e microesclerócios em sementes infetadas. Microesclerócios são produzidos em colmos secos de cultivares suscetíveis, sendo a sua sobrevivência maior em restos culturais mantidos na superfície do solo. O patógeno é altamente variável, conforme demonstrado através da virulência em plantas diferenciadoras e de marcadores moleculares. As implicações desta variabilidade no desenvolvimento de estratégias de manejo desta doença, através da resistência genética são aspectos discutidos neste trabalho.

Avaliação de fungicidas para o controle de Colletotrichum gloeosporioides em cebola

A antracnose foliar causada por Colletotrichum gloeosporioides é a doença mais importante da cebolicultura (Allium cepae) em Guidoval e Guiricema, MG. Para controle da doença, utilizam-se intensivamente fungicidas, principalmente benomil (Ben), o que tem sido pouco efetivo. Neste trabalho, testou-se a hipótese da existência de isolados de C. gloeosporioides resistentes ao Ben e se comparou a eficiência de diferentes fungicidas no controle do patógeno. Nas concentrações testadas, Ben impediu o crescimento micelial do fungo, e a germinação de conídios decresceu com o aumento da concentração do produto. Independente do isolado do patógeno e da concentração dos fungicidas, ocorreu maior inibição da germinação com Tiofanato metílico (Tm) + Mancozeb (Man), Man, Clorotalonil (Clo), Ben+Man, TM+Clo, Captan (Cap) e Procloraz (Pro). Excetuando-se Clo, os demais tratamentos inibiram totalmente o crescimento micelial do fungo. Em casa de vegetação, pulverizaram-se os diferentes tratamentos em plantas de cebola 'Texas Early Grano 502' (TEG502) e 24 h após inoculou-se o patógeno. Maior redução na intensidade da doença ocorreu com TM+Man, TM+Clo, Ben+Man, Man, Cap, Clo, Ben e Pro. Em plantas de TEG502, pulverizaram-se diferentes fungicidas imediatamente após, dois, quatro ou seis dias após inoculação do patógeno. Na pulverização imediatamente após a inoculação, Ben e Pro foram os mais eficientes; nas demais épocas, nenhum fungicida reduziu a severidade da doença.

Pattern Analysis of Sorghum Genotype × Environment Interaction for Leaf, Panicle, and Grain Anthracnose in Mali.

Resistance to anthracnose, caused by Colletotrichum graminicola, in sorghum was identified through field screening at two locations (Samanko and Longorola) in Mali. The occurrence and progress of anthracnose were monitored on 19 sorghum lines plus resistant and susceptible checks in the 1996 to 1998 rainy seasons. Foliar anthracnose severity was assessed at regular intervals throughout the season. Area under the disease progress curve (AUDPC) was calculated for each genotype. Anthracnose severity was also evaluated on the peduncle, rachis and glumes, panicle, and grain. For the characters under study, the site × year and site × year × line interactions accounted for the genotype × environment interactions. Pattern analysis was applied to the environment-standardized matrix of genotype × environment means to analyze these interactions and elucidate genotypic adaptation. None of the lines was completely (hypersensitive) resistant to the disease, but 12 showed high levels of stable resistance to both foliar and panicle anthracnose. Only one was moderately susceptible to both forms of the disease. In addition to identifying varieties that can be grown in zones to which they are adapted, additional genotypes were identified that can serve as sources of resistance in regional breeding programs.

Molecular identification of Colletotrichum gloeosporioides causing yam anthracnose in Nigeria

Four forms of Colletotrichum representing three distinct virulence phenotypes were found associated with foliar anthracnose of yam in Nigeria: the highly virulent (= severity of disease) slow‐growing grey (SGG); the moderately virulent fast‐growing salmon (FGS); the weakly virulent fast‐growing grey (FGG); and the moderately virulent fast‐growing olive (FGO) morphotype. Isolates of the four forms were identified as C. gloeosporioides, based on morphology. The reaction of monoconidial cultures on casein hydrolysis medium (CHM), PCR‐RFLP and sequence analysis of the internal transcribed spacer region of the ribosomal DNA (ITS1‐5·8S‐ITS2) were used to establish the identity of the yam anthracnose pathogen(s). All yam isolates were distinguished from C. acutatum by the absence of protease activity on CHM. On ITS PCR and enzymatic digestion of PCR products, all FGS, FGO and SGG isolates produced RFLP patterns identical to those of C. gloeosporioides reference isolates, while FGG isolates revealed unique ITS RFLP banding patterns. Sequence analysis of the ITS1 region and of the entire ITS region revealed that SGG, FGS and FGO isolates were highly similar (98–99% nucleotide identity) and showed 97–100% identity to C. gloeosporioides. Less than 93% similarity of these fungal isolates to reference C. acutatum and C. lindemuthianum isolates was observed. The molecular study confirmed that foliar anthracnose of yam is caused by C. gloeosporioides. While a high similarity was found among most C. gloeosporioides fungi from yam, isolates of the FGG form did not cluster with any previously described Colletotrichum species, and probably represent a distinct species.

Controle genético da antracnose foliar em milho

Em experimento realizado em casa de vegetação com quatro linhagens e dois híbridos de milho (Zea mays), verificou-se variabilidade para resistência a queima foliar causada por Colletotrichum graminicola avaliada por meio das variáveis área total de lesão por planta (ATL), área média de lesão (AML), comprimento total de lesão por planta (CTL), comprimento médio de lesão (CML) e número de lesões por planta (NL). Quando dois isolados de C. graminicola foram inoculados nessas linhagens e em híbridos de milho não foi verificada interação diferencial entre isolados e genótipos do hospedeiro. Em estudos de herança da resistência, as linhagens genitoras L186 e L64 comportaram-se como suscetível e resistente, respectivamente, ao passo que o híbrido L186 x L64 apresentou resistência. A análise das freqüências observadas de plantas resistentes e suscetíveis na população F2 resultantes da autofecundação do híbrido, indicou ocorrência de herança monogênica com dominância completa para o alelo que confere resistência à doença.

Variation among foliar isolates of Colletotrichum sublineolum of sorghum in Nigeria

Foliar anthracnose, caused by Colletotrichum sublineolum, is a major disease of sorghum in Nigeria. Pathogenic diversity was studied among foliar isolates of C. sublineolum collected from sorghum cultivars in farmers fields in the Sahel, Sudan, northern Guinea and southern Guinea zones of Nigeria during the 1997 crop season. For the study a total of 50 isolates were identified based on typical symptom types and cultural characteristics. On the basis of growth in culture and morphological characteristics, the isolates were classified into nine morphological groups (MGs). Pathogenicity tests were done on a set of known sorghum differential lines by inoculating pot-grown seedlings in a greenhouse. On the basis of disease reaction and disease severity scores on the differential lines, the 50 isolates were classified into seven pathogenic groups (PGs). Sixteen representative isolates of the MGs and the PGs were further evaluated for virulence and aggressiveness on the differential lines and were classified into five distinct groups using the centroid method of cluster analysis. The existence of five races of C. sublineolum in major sorghum growing zones of Nigeria is suggested.

Dogwood Anthracnose (Discula destructiva): Effects of and Consequences for Host (Cornus florida) Demography

Abstract Dogwood anthracnose is a disease caused by Discula destructiva, a fungus of probable exotic origin that is a serious threat to natural populations of Cornus florida in the eastern United States. This epidemic provides an interesting opportunity for the study of the ecology of a highly virulent pathogen and its host. We present evidence that dead dogwoods tend to have larger neighboring conspecifics than do living dogwoods, that smaller trees have higher levels of foliar infection than larger trees and that in trees of reproductive size, higher levels of anthracnose infection are associated with lower fruit production. In contrast to our data on mortality, proximity to conspecifics did not account for significant variation in the severity of foliar anthracnose infection among live dogwoods. These patterns of infection suggest that D. destructiva could severely impair the ability of this important component of the forest understory to regenerate itself.

Comparison of disease progress curves for yam anthracnose (Colletotrichum gloeosporioides)

The incidence and severity of foliar anthracnose, caused by Colletotrichum gloeosporioides, were recorded for seven fields of yams (Dioscorea alata var. White Lisbon) in Barbados over a complete growing season. A scoring system was developed to estimate severity, placing the emphasis on the earliest, least visible stages of the disease. Disease progress curves were described in terms of the logistic model and the effects of site and leaf age on the parameters as well as on disease at tuber bulking were analysed. Differences between sites indicated that disease was more severe in areas of higher rainfall. In severe epidemics, disease predominated on young leaves. Fungicides applied sufficiently early can delay the onset of disease and slow the rate of disease progress.