Leaf Spot Blight Research Articles

Phenotypic Variability in Resistance to Anthracnose, White, Brown, and Blight Leaf Spot in Cassava Germplasm.

Despite fungal diseases affecting the aerial parts of cassava (Manihot esculenta Crantz) and causing significant yield losses, there is a lack of comprehensive studies assessing resistance in the species' germplasm. This study aimed to evaluate the phenotypic diversity for resistance to anthracnose disease (CAD), blight leaf spot (BliLS), brown leaf spot (BLS), and white leaf spot (WLS) in cassava germplasm and to identify genotypes suitable for breeding purposes. A total of 837 genotypes were evaluated under field conditions across two production cycles (2021 and 2022). Artificial inoculations were carried out in the field, and data on yield and disease severity were collected using a standardized rating scale. The top 25 cassava genotypes were selected based on a selection index for disease resistance and agronomic traits. High environmental variability resulted in low heritabilities (h2) for CAD, WLS, and BLS (h2 = 0.42, 0.34, 0.29, respectively) and moderate heritability for BliLS (h2 = 0.51). While the range of data for disease resistance was narrow, it was considerably wider for yield traits. Cluster analysis revealed that increased yield traits and disease severity were associated with higher scores of the first and second discriminant functions, respectively. Thus, most clusters comprised genotypes with hybrid characteristics for both traits. Overall, there was a strong correlation among aerial diseases, particularly between BLS and BliLS (r = 0.96), while the correlation between CAD and other diseases ranged from r = 0.53 to 0.58. Yield traits showed no significant correlations with disease resistance. Although the mean selection differential for disease resistance was modest (between -2.31% and -3.61%), selection based on yield traits showed promising results, particularly for fresh root yield (82%), dry root yield (39%), shoot yield (49%), and plant vigor (26%). This study contributes to enhancing genetic gains for resistance to major aerial part diseases and improving yield traits in cassava breeding programs.

Development of prediction equations for early blight leaf spot on tomato under different fungicides treatments

Field experiment was conducted during 2008-09 and 2009-10 to study the influence of different weather factors on the development of early blight on tomato caused by Alternaria solani (Ell. and Mart.) under six different fungicidal treatments and also to compare the two transformation models viz. Logit and Gompertz through which the disease progress curve move over time. Here, different prediction equationswere developed for each fungicide treatment separately through step down multiple regression analysis which showed that different meteorological factors having different influence on disease severity and these were done after Logistic and Gompertz transformation of the realized observed value of the disease severity (expressed as AUDPC).Results revealed that among the six fungicides tested Mancozeb was superior in controlling the disease severity with the lowest (AUDPC: 97.34 and 95.74) for the two experimental years respectively. Linearization of area under disease progress curve (AUDPC) following the two models (Logit and Gompit) showed that Gompit fit better than Logit for the prediction of early blight disease severity and this was confirmed by the low standard error estimate value of Gompertz. The co-efficient of determination value (R2) showed thatvariation in disease severity can be explained up to 77 percent (with an exception 85percent in case of Mancozeb application in 2009-10) in Logistic as well as 88 percent in Gompertz with combined effect of the weather variables included in the present study. Among the seven meteorological factors considered only average temperature,RH and total rainfall were found to act positively and significantly,whereas, bright sunshine hours were found to have negative significant effect on early blight disease severity on tomato. These situations were observed in both the transformation models but vary with in the treatments and with in the years.

Standard area diagrams to aid assessments of the severity of blight leaf spot symptoms on cassava leaves

A standard area diagram (SAD) set to assess the severity of blight leaf spot (BliLS) caused by Passalora vicosae on cassava leaves was developed and validated. The proposed SAD set includes diagrams illustrating six leaves with different severities of blight leaf spot (5, 10, 20, 30, 40 and 50%). The SAD set was validated by ten raters with no previous experience in disease assessment. Lin’s concordance correlation analysis of estimated versus actual disease severity showed that the statistical parameters (υ, u, r and ρc) were improved when raters were aided by the SADs. Overall agreement measured by Lin’s concordance correlation coefficient (ρc) increased from 0.70 to 0.82, without and with SADs, respectively. Similarly, by analysing the coefficient of determination (R2) and intra-class correlation coefficient (ρ), the estimates of severity were more reliable by using the SADs. The proposed SAD set increases the accuracy and reliability of estimates of the severity of BliLS in cassava and therefore is a valuable aid for research on the epidemiology, breeding for resistance, fungicide screening and pathotype characterization.

Inbreeding depression for severity caused by leaf diseases in cassava

The use of inbred populations in cassava has been little exploited by breeding programs. Therefore, the objective of this study was to evaluate the effects of inbreeding depression (ID) in S1 families for resistance to leaf diseases and to select transgressive individuals. Five cassava varieties (Cascuda, BRS Formosa, Fecula Branca, Mani-Branca and BRS Mulatinha) were selfed, and the families S0 and S1 were evaluated for severity caused by cassava white leaf spot (CWLS), cassava brown leaf spot (CBRLS) and cassava blight leaf spot (CBLS) under field conditions. Differences among S1 families and between generations (S0 × S1) were identified for all diseases. The wide variation of CWLS (2.16–93.38%), CBRLS (15.31–69.39%) and CBLS (15.70–53.64%) severity highlighted the occurrence of ID in S1 families, in which the average was 12.61% (CWLS), 13.72% (CBRLS) and 15.92% (CBLS). Mean estimates of homozygous loci contribution (µ + α) were of higher magnitude compared with heterozygous loci contribution (δ) for all diseases and families. Even in the occurrence of ID, it was possible to select resistant S1 individuals, in a way that the reduction in the severity of diseases based on the selection of the five most resistant individuals was 75.80% (S1-BRS Formosa) for CWLS, 38.64% (S1-Cascuda) for CBRLS and 33.51% (S1-Cascuda) for CBLS. These results demonstrate the predominance of additive genetic effects and the possibility to select transgressive individuals in S1 families for increasing the resistance to cassava leaf diseases.

Assessment of major cassava diseases in Togo in relation to agronomic and environmental characteristics in a systems approach

A cassava disease survey was conducted in four agroecological zones of Togo. High incidences of cassava bacterial blight, cassava mosaic disease and cercosporioses were observed across ecozones, while anthracnose disease was rare. Bacterial blight field incidences of 90.5% in the dry savanna zone, 70% in the forest savanna transition zone, 64% in the wet savanna zone and 52.6% in the forest zone, were recorded, with plant incidences ranging from 27.4% in the forest zone to 72.7% in the dry savanna zone. Mosaic disease field incidences were nearly 100% in all the ecozones and high plant incidences up to 86.9% were found. Cercospora leaf diseases – brown leaf spot, blight leaf spot and white leaf spot - occurred in all the ecozones with incidences ranging from 68 to 100%. Negative correlations between bacterial blight and mosaic disease, and between mosaic disease and white leaf spot were found, while brown leaf spot and blight leaf spot, brown leaf spot and white leaf spot, and blight leaf spot and white leaf spot were positively correlated. Field incidence of bacterial blight was positively correlated with plant age, ecozones - higher severity in dryer ecozones (P < 0.01), and weed density (P < 0.05). Further significant, but negative correlations occurred between bacterial blight and cercospora brown leaf spot on the other hand and vegetation type in the surroundings field (number of trees) (P < 0.05).Cercospora brown leaf spot was also significantly negatively associated with the number of crops in a field (intercropping) (P < 0.05), and Cercospora white leaf spot with sandy soils (P < 0.01). Results indicate that agroecological factors such as ecozone, plant age, weed density, vegetation type, intercropping and soil type and moisture influence disease occurrence and should be considered when developing integrated control measures. Key words: Blight leaf spot, brown leaf spot, cassava bacterial blight, cassava mosaic disease, white leaf spot, Cercospora henningsii, Cercospora caribaea, Cercospora vicosae, Colletotrichum gloeosporioides f. sp. manihotis, Manihot esculenta, Xanthomonas axonopodis pv. manihotis.