Phaeosphaeria Leaf Spot Research Articles

Benefits of maize resistance breeding and chemical control against northern leaf blight in smallholder farms in South Africa

Maize underpins food security in South Africa. An annual production of more than 10 million tons is a combination of the output of large-scale commercial farms plus an estimated 250 000 ha cultivated by smallholder farmers. Maize leaves are a rich source of nutrients for fungal pathogens. Farmers must limit leaf blighting by fungi to prevent sugars captured by photosynthesis being ‘stolen’ instead of filling the grain. This study aimed to fill the knowledge gap on the prevalence and impact of fungal foliar diseases in local smallholder maize fields. A survey with 1124 plant observations from diverse maize hybrids was conducted over three seasons from 2015 to 2017 in five farming communities in KwaZulu-Natal Province (Hlanganani, Ntabamhlophe, KwaNxamalala) and Eastern Cape Province (Bizana, Tabankulu). Northern leaf blight (NLB), common rust, Phaeosphaeria leaf spot, and grey leaf spot had overall disease incidences of 75%, 77%, 68% and 56%, respectively, indicating high disease pressure in smallholder farming environments. NLB had the highest disease severity (LSD test, p<0.05). A yield trial focused on NLB in KwaZulu-Natal showed that this disease reduced yields in the three most susceptible maize hybrids by 36%, 71% and 72%, respectively. Eighteen other hybrids in this trial did not show significant yield reductions due to NLB, which illustrates the progress made by local maize breeders in disease resistance breeding. This work highlights the risk to smallholder farmers of planting disease-susceptible varieties, and makes recommendations on how to exploit the advances of hybrid maize disease resistance breeding to develop farmer-preferred varieties for smallholder production.
 Significance:
 
 Northern leaf blight, grey leaf spot, Phaeosphaeria leaf spot and common rust diseases were widespread in KwaZulu-Natal and Eastern Cape smallholder maize fields where fungicides were not applied.
 NLB was the most severe maize leaf disease overall.
 NLB caused maize leaf blighting, which reduced grain yields by 36–72% in susceptible maize hybrids.
 Maize resistance breeding has produced locally adapted hybrids that do not have significant yield losses under NLB disease pressure.

Testing the spectral resolutions of the new multispectral sensors for detecting <i>Phaeosphaeria</i> leaf spot (PLS) infestations in maize crop

Maize is one of the most important subsistence and commercial crops in the world. In Africa, it is regarded as one of the most popular food crops. Recently however, significant losses due to Phaeosphaeria leaf spot (PLS) infestation have been reported. Therefore, techniques for early detection of PLS infestation are valuable for mitigating maize yield losses. Recently, remotely sensed datasets have become valuable in crop assessment. In this study, we compared the performance of commonly used higher spatial resolution sensors (WorldView, Quickbird, Sentinel series 2, RapidEye and SPOT 6) resampled to field hyperspectral remotely sensed data in detecting early PLS infestation. Canopy training spectra were collected on leaves with signs of early infestation and healthy leaves spectral characteristics used for comparison. Training data was collected in 2013 growing season while test data was collected under similar conditions in 2014. The Random Forest algorithm was used to establish the Kappa and overall, user and producer's accuracies. Results showed that the RapidEye sensor with an 86.96% and Kappa value of 0.76 performed better than the rest of the sensors while the Red, Yellow and Red-Edge bands were most useful for detecting early PLS infestation. The value of the RapidEye sensor in detecting early PLS infestation can be attributed to the optimally centred Red Red-Edge bands sensitive to changes in chlorophyll content, a consequent of PLS infestation on maize leaves. The study provides valuable insight on the value of existing sensors, based on their sensor characteristics in detecting early PLS infestation.

INFLUENCE OF FOLIAR APPLICATION OF FUNGICIDES TO CONTROLPHAEOSPHAERIA LEAF SPOT IN MAIZE

The objective of this study was to evaluate the effectiveness of foliar application of fungicides to control Phaeosphaeria leaf spot and consequently on the features of the maize crop productivity in the crop year 2014/15. The experimental design was randomized blocks, in a 2x4 factorial scheme, with two hybrids, three treatments + a control in four replications. The treatments were based on the following fungicides (g i.a L-1): epoxiconazole + pyraclostrobin (333 + 167); cyproconazole + picoxystrobin (80 + 200); azoxystrobin + benzovinliflupyr (300 + 150), which were applied in BG7051H and BG7060HR hybrids with knapsack/ manual sprayer, pressurized with CO2 in the phenological stage of culture V8, 70 days after seeding (70 DAS). The experimental plot had four rows spaced at 0.50 m and length of 10.0 m, resulting in a total area of 20 m2 and useful area of 8 m2, which held the white spot severity assessments, grain yield (kg ha-1) and thousand grain weight (g). Significant differences were observed for all fungicides compared to the control. The control alone had the highest severity index compared to the other treatments. Regarding the thousand grain weight and productivity (kg ha-1), the azoxystrobin + solatenol treatment was superior to the control and other treatments with fungicides, with a higher production of grain compared to the control.

Detecting the Early Stage of Phaeosphaeria Leaf Spot Infestations in Maize Crop Using In Situ Hyperspectral Data and Guided Regularized Random Forest Algorithm

Phaeosphaeria leaf spot (PLS) is considered one of the major diseases that threaten the stability of maize production in tropical and subtropical African regions. The objective of the present study was to investigate the use of hyperspectral data in detecting the early stage of PLS in tropical maize. Field data were collected from healthy and the early stage of PLS over two years (2013 and 2014) using a handheld spectroradiometer. An integration of a newly developed guided regularized random forest (GRRF) and a traditional random forest (RF) was used for feature selection and classification, respectively. The 2013 dataset was used to train the model, while the 2014 dataset was used as independent test dataset. Results showed that there were statistically significant differences in biochemical concentration between the healthy leaves and leaves that were at an early stage of PLS infestation. The newly developed GRRF was able to reduce the high dimensionality of hyperspectral data by selecting key wavelengths with less autocorrelation. These wavelengths are located at 420 nm, 795 nm, 779 nm, 1543 nm, 1747 nm, and 1010 nm. Using these variables (n=6), a random forest classifier was able to discriminate between the healthy maize and maize at an early stage of PLS infestation with an overall accuracy of 88% and a kappa value of 0.75. Overall, our study showed potential application of hyperspectral data, GRRF feature selection, and RF classifiers in detecting the early stage of PLS infestation in tropical maize.

ETIOLOGY OF PHAEOSPHAERIA LEAF SPOT DISEASE OF MAIZE

Different fungal species and the bacterium Pantoea ananatis (Pa) have been reported as etiological agents of Phaeosphaeria leaf spot (PLS) disease. This work aimed at using molecular identification of the fungi and bacteria occurring in PLS and its etiology. Genomic DNAs from (i) pool of each of the four stages of PLS lesions; (ii) bacteria and fungi isolated from lesions of natural PLS (NPLS) and artificial injuries (AI); (iii) fungi isolated from lesions obtained from plants inoculated with Pa in greenhouse (GH) were used in PCR with universal primers for bacteria and fungal rRNA genes, and species-specific primers for Pa. Bacterial amplicons were observed at all stages of lesions and fungal amplicons in stages 3 and 4. Bacterial amplicons of pooled NPLS lesions were from Pa while fungal amplicons were from Phaeosphaeria sp. and Phoma sp. Bacteria from NPLS, GH and AI lesions were identified as Pa, Pa and Bacillus subtilis, respectively, while the fungi were Epicoccum nigrum, Leptosphaeria sacchari, Cochliobolus geniculatus, Pithomyces chartarum, Alternaria alternata, A. ricini, Gibberella intricans, G. fujikuroi, Phaeosphaeria sp., P. avenaria, Phoma sp., Phyllosticta sp., Sarocladium strictum, Glomerella graminicola, and Cochliobolus heterostrophus. Symptoms of PLS decreased by 90% with the use of oxytetracycline in foliar treatment of maize plants in the field, and its addition to culture medium completely inhibited the growth of Pa. The results strongly show that Pa is the causal agent of PLS disease and different species of opportunistic fungi appear late in the necrotic stages of lesions caused by Pa.

English

Diseases and pests are among the major constraints limiting maize productivity in the smallholder (SH) farming sector of sub-Saharan Africa. The objectives of this study were therefore, to determine how SH farmers perceive and cope with diseases and pests, identify with farmersother constraints to maize production. Data were collected from three villages of KwaZulu-Natal (KZN) province in South Africa using surveys and participatory rural appraisal (PRA) methodology. Local maize varieties were further evaluated for disease resistance and grain yield potential at two locations in KZN. Cob rots, grey leaf spot (GLS), maize streak virus (MSV), northern leaf blight (NLB), phaeosphaeria leaf spot (PLS) and common rust were the main diseases identified by farmers, but their incidence was low and occurrence infrequent. More than 75% of the farmers did not control both insect pests and diseases, while the rest used chemicals or everyday household remedies for control. Stalk borers and cutworms were the most prevalent insect pests, where as drought, excessive rains, hail storms, and soil fertility were the most important abiotic constraints identified. Field trial results of local varieties indicated high yield potential and genetic variability for disease resistance to PLS, GLS, and NLB. These findings suggest that; if the main production constraints are addressed, farmers could realize high yields from their local varieties. Breeding opportunities, therefore, exist for incorporating resistance or tolerance to these stresses into the local varieties. Key words: Maize, diseases, pests, constraints, breeding, smallholder farmers.

Escala diagramática para avaliação da severidade da mancha branca em milho

Escalas diagramáticas são importantes ferramentas para padronizar as estimativas da severidade de doenças em plantas. O objetivo deste trabalho foi desenvolver e validar uma escala diagramática para avaliação da severidade da mancha branca ou mancha de Phaeosphaeria em folhas de milho. A escala proposta ilustra os níveis de 1; 3; 6; 13; 25; 43; 63 e 79 % de severidade no terço médio de imagens de folhas de milho. A validação da escala foi realizada com o auxílio de dez avaliadores (cinco sem experiência e cinco com experiência), os quais estimaram a severidade da doença em 50 folhas de milho com sintomas da mancha branca, com e sem a utilização da escala. Análises de regressão foram utilizadas para avaliar a precisão e acurácia. A avaliação da acurácia indicou que sem o uso da escala sete avaliadores apresentaram desvios sistemáticos nas estimativas, enquanto que com o uso da escala foram apenas dois avaliadores. A precisão representada pelo coeficiente de determinação (r² ) das regressões, variou de 0,86 a 0,95 para os avaliadores inexperientes sem escala e 0,91 a 0,98 com escala. Os avaliadores experientes apresentaram r² variando de 0,91 a 0,95 sem escala e 0,94 e 0,98 com escala. A escala diagramática proposta melhorou a precisão e a acurácia das estimativas, independente da experiência do avaliador.

Progeny evaluation for resistance to Phaeosphaeria leaf spot in tropical maize

Maize breeding programmes in Brazil and elsewhere seek reliable methods to identify genotypes resistant to Phaeosphaeria leaf spot. The area under the disease progress curve (AUDPC) is an accurate method to evaluate the severity of foliar diseases. However, at least three data points are required to calculate the AUDPC, which is unfeasible when there are thousands of genotypes to be assessed. The aim of this work was to estimate the heritability of disease resistance, evaluate disease severity at different times using a nine-point scale in comparison to the AUDPC, and establish the most suitable phenological period for disease assessment. A repeated experiment was conducted in a 11 × 11 lattice experimental design with three replications. Disease assessments were carried out at flowering, 15 and 30 days post-anthesis for the parental lines DS95, DAS21, the F1 generation and 118 F2:3 progenies. Then, the AUDPC was obtained and results compared with the single-point evaluations used to calculate it. Individual and joint analyses of variance were conducted to obtain heritabiliy estimates. The assessments performed after the flowering stage gave higher estimates of heritability and correlation with AUDPC. We concluded that one assessment between the 15th and 30th day after flowering could provide enough information to distinguish maize genotypes for their resistance to Phaeosphaeria leaf spot under tropical conditions.

Combining ability analysis for Phaeosphaeria leaf spot resistance and grain yield in tropical advanced maize inbred lines

Phaeosphaeria leaf spot (PLS) disease (causal agent Phaeosphaeria maydis (Henn.) Rane, Payak & Renfro) of maize is increasing in importance in sub-Saharan Africa (SSA). However, there is still limited information on the combining ability for disease resistance of the germplasm that are adapted to African environments. Evaluating combining ability effects and their interactions with the environment would provide valuable information that can be used in the development of cultivars that are resistant to PLS. This study was therefore conducted to determine the combining ability, gene action and the relationship between grain yield and PLS disease severity among selected tropical advanced maize inbred lines. Forty five F 1 hybrids were generated by crossing 10 inbred lines in a half diallel mating scheme. The 45 hybrids along with the ten inbred parents were evaluated in four environments, with two replications each between 2007 and 2009. General and specific combining ability (GCA and SCA) effects were highly significant ( P ≤ 0.001) for PLS, grain yield and days to anthesis. GCA effects accounted for 66–90% and SCA effects for 10–34% of the variation in the hybrids for PLS resistance, grain yield and days to anthesis. This indicated predominance of additive over non-additive gene action for the three traits in these inbred lines. The resistant inbred lines to PLS were A1220-4, N3-2-3-3, CML312, MP18 and CML488. These lines had good combining ability for PLS resistance and contributed towards resistance in their crosses. In general, resistant hybrids involved a susceptible and a resistant parent, where at least one of the parents had a negative GCA effect. In addition, lines A1220-4 and CML312 contributed towards high yield and were late maturing. Inbred line CZL00009 conferred genes for early maturity. Linear regression analysis indicated that grain yield of maize was suppressed by about 250 kg ha −1 per each increase in PLS disease severity score, underscoring the need to control the disease. Significant ( P < 0.01), negative correlations ( r = −0.29 to −0.43) between grain yield and PLS severity were also detected. This showed the potential of PLS to reduce yield when favourable conditions for disease development are present. By and large, highly significant additive gene action implied that progress would be made through selection. Although non-additive effects were small (±10%), observation of dominance effects which were associated with reduced disease levels in some hybrids may be exploited in developing single cross maize hybrids among these inbreds when one parent is resistant.

QTL mapping for reaction to Phaeosphaeria leaf spot in a tropical maize population

Phaeosphaeria leaf spot (PLS) is an important disease in tropical and subtropical maize (Zea mays, L.) growing areas, but there is limited information on its inheritance. Thus, this research was conducted to study the inheritance of the PLS disease in tropical maize by using QTL mapping and to assess the feasibility of using marker-assisted selection aimed to develop genotypes resistance to this disease. Highly susceptible L14-04B and highly resistant L08-05F inbred lines were crossed to develop an F(2) population. Two-hundred and fifty six F(2) plants were genotyped with 143 microsatellite markers and their F(2:3) progenies were evaluated at seven environments. Ten plants per plot were evaluated 30 days after silk emergence following a rating scale, and the plot means were used for analyses. The heritability coefficient on a progeny mean basis was high (91.37%), and six QTL were mapped, with one QTL on chromosomes 1, 3, 4, and 6, and two QTL on chromosome 8. The gene action of the QTL ranged from additive to partial dominance, and the average level of dominance was partial dominance; also a dominance x dominance epistatic effect was detected between the QTL mapped on chromosome 8. The phenotypic variance explained by each QTL ranged from 2.91 to 11.86%, and the joint QTL effects explained 41.62% of the phenotypic variance. The alleles conditioning resistance to PLS disease of all mapped QTL were in the resistant parental inbred L08-05F. Thus, these alleles could be transferred to other elite maize inbreds by marker-assisted backcross selection to develop hybrids resistant to PLS disease.

Genetic resistance of new popcorn hybrids to foliar diseases

The level of genetic resistance to foliar diseases in 54 new simple popcorn hybrids was evaluated in field conditions. Two trials in a randomized complete block design were conducted in the 2006/07 growing season. The disease intensity in genotypes was compared by the area under the disease progress curve, expressing the severity. Significant differences (P ≤ 0.01) of resistance to southern rust (Puccinia polysora) northern leaf blight (Exserohilum turcicum) and Phaeosphaeria leaf spot (complex Phaeosphaeria maydis / Pantoea ananas) were observed. The cluster analysis detected different resistance levels to the series of leaf diseases. In 12 of the new hybrids the resistance level to the set of diseases was similar as in the commercial hybrid IAC 112, considered the best reference for leaf disease resistance among commercial popcorn genotypes.

Desempenho agronômico de novos híbridos de milho-pipoca no Noroeste do Estado do Paraná

In maize, variations on agronomic performance in distinct environments are common and more evident for one-way hybrids. The objective of this work was to evaluate the performance of new one-way popcorn hybrids developed by the Breeding Program of the State University of Maringa, in northwestern Parana. The characteristics evaluated were: grain yield, popping expansion, plant height, ear height and the intensity of Southern rust ( Puccinia polysora ), Northern leaf blight ( Exserohilum turcicum ), and phaeosphaeria leaf spot ( Phaeosphaeria maydis/Pantoea ananas ). The assay was carried out into randomized block design with three replications in Maringa, state of Parana, Brazil, during the 2006/2007 cropping season. The treatments were 27 simple popcorn hybrids obtained from S5 endogamic inbreds and commercial cultivars BRS Angela, IAC 112 and Jade. Significant differences (p ≤ 0.05) were verified among hybrids for all characteristics. The hybrid from C and K inbreds presented better indexes for grain yield and popping expansion; and was susceptible to Southern rust. About diseases, the hybrids from the L, H and K inbreds showed greater resistance levels to Southern rust, and hybrids from G inbred had better Northern leaf blight resistance. Hybrids from L and M inbreds showed better phaeosphaeria leaf spot resistance.

Localization of Pantoea ananatis inside lesions of maize white spot disease using transmission electron microscopy and molecular techniques

The etiological agent of maize white spot (MWS) disease has been a subject of controversy and discussion. Initially the disease was described as Phaeosphaeria leaf spot caused by Phaeosphaeria maydis. Other authors have suggested the existence of different fungal species causing similar symptoms. Recently, a bacterium, Pantoea ananatis, was described as the causal agent of this disease. The purpose of this study was to offer additional information on the correct etiology of this disease by providing visual evidence of the presence of the bacterium in the interior of the MWS lesions by using transmission electron microscopy (TEM) and molecular techniques. The TEM allowed visualization of a large amount of bacteria in the intercellular spaces of lesions collected from both artificially and naturally infected plants. Fungal structures were not visualized in young lesions. Bacterial primers for the 16S rRNA and rpoB genes were used in PCR reactions to amplify DNA extracted from water-soaked (young) and necrotic lesions. The universal fungal oligonucleotide ITS4 was also included to identify the possible presence of fungal structures inside lesions. Positive PCR products from water-soaked lesions, both from naturally and artificially inoculated plants, were produced with bacterial primers, whereas no amplification was observed when ITS4 oligonucleotide was used. On the other hand, DNA amplification with ITS4 primer was observed when DNA was isolated from necrotic (old) lesions. These results reinforced previous report of P. ananatis as the primary pathogen and the hypothesis that fungal species may colonize lesions pre-established by P. ananatis.

Controle genético da resistência à mancha-de-Phaeosphaeria em milho

A mancha-de-Phaeosphaeria, de ampla ocorrência no Brasil, tem causado expressiva redução na produtividade da cultura do milho no país. Por isso, o desenvolvimento de híbridos resistentes à doença é um dos principais objetivos dos programas de melhoramento genético da cultura. Informações sobre o controle genético da resistência à doença são necessárias para que os programas de melhoramento sejam eficientes. O objetivo deste estudo foi determinar o controle genético da resistência à mancha-de-Phaeosphaeria em milho a partir da avaliação das médias de gerações oriundas de dois cruzamentos entre linhagens resistentes (DAS95 ou DAS72) com uma suscetível (DAS21), sob condições de infecção natural da doença. Os ensaios foram conduzidos em Indianópolis (MG), em duas épocas de semeadura, outubro e novembro de 2000. Foi utilizado o delineamento experimental blocos ao acaso com três repetições. A avaliação para resistência foi realizada 30 dias após o florescimento com auxílio de uma escala diagramática de porcentagem de tecido foliar total da planta afetado pela doença. As médias da severidade da doença dos genitores e das gerações F1, F2, RCP1 e RCP2 foram analisadas segundo o modelo de MATHER &amp; JINKS (1971). A variação genética devida a efeitos aditivos foi de 73 a 84%, enquanto, devido aos efeitos dominantes, foi de 13 a 23%. Foi evidenciada a predominância dos efeitos gênicos aditivos sobre dominantes em ambas as populações estudadas. Os valores da herdabilidade foram altos, variando de 61 a 88%. Os resultados indicaram condições favoráveis ao melhoramento genético visando àresistência à mancha-de-Phaeospheria nas populações estudadas.

Modelo agrometeorológico regional para estimativa da severidade da mancha de Phaeosphaeria em milho safrinha no Estado de São Paulo, Brasil

O milho safrinha, cultivado no outono-inverno, no Estado de São Paulo, tem apresentado com freqüência significativas reduções de produtividade devidas à mancha foliar de Phaeosphaeria. Como estratégia para minimizar esses danos, os modelos agrometeorológicos de previsão de doenças podem auxiliar no planejamento e na tomada de decisões para o controle da doença na cultura. Dessa forma, este trabalho tem como objetivo o desenvolvimento e teste de modelos agrometeorológicos para previsão da porcentagem da área foliar afetada pela doença (%AFA) em função do acúmulo diário de temperatura média e de chuva, em escala regional. Para tanto, foram utilizadas informações sobre a severidade da mancha de Phaeosphaeria em 158 cultivares de milho com diferentes níveis de resistência, durante a safrinha, em 14 diferentes localidades do Estado de São Paulo. Modelos foram desenvolvidos para cultivares suscetíveis, moderadamente resistentes e resistentes e os resultados indicaram haver boa performance na estimativa da %AFA, com altos valores de coeficiente de determinação (0,92; 0,81 e 0,83 respectivamente) e índice de concordância de Willmott (0,98; 0,87; 0,94 respectivamente), além de teste F significativo a 1% de probabilidade para todos os casos. Os resultados também permitiram sugerir que a chuva e a temperatura são os fatores preponderantes para a ocorrência da mancha de Phaeosphaeria no Estado de São Paulo. Esses modelos têm a vantagem de utilizar variáveis usualmente obtidas em estações meteorológicas e poderão ser empregados em sistemas de alerta fitossanitários para monitoramento da doença no Estado.

Gene action determining Phaeosphaeria leaf spot disease resistance in experimental maize hybrids

Phaeosphaeria leaf spot (Phaeosphaeria maydis Henn.) has the potential to cause substantial yield losses in maize. Maize is grown by small-scale farmers without fungicides; hence there is need to breed for resistance in regionally adapted germplasm. Little information about the gene action determining Phaeosphaeria leaf spot disease (PLS) resistance in African maize germplasm is currently available. This study was therefore conducted to determine the gene action controlling resistance to PLS in African maize germplasm. Seventy-two experimental hybrids were generated in eight sets according to a North Carolina Design II mating scheme. Experimental and check hybrids were evaluated in an 8 × 8 simple lattice design during the 2003/4 season, and in an 8 × 10 α-lattice design, with two replications, during the 2004/5 seasons at the Cedara and Rattray Arnold Research Stations, in South Africa and Zimbabwe, respectively. There was significant variation among hybrids for resistance. General combining ability (GCA) due to both male and female inbred parents were highly significant (P<0.01), whereas specific combining ability effects were not significant for PLS scores, indicating that mainly additive gene action conditioned PLS resistance in experimental hybrids. Significant differences between male and female GCA variances, suggested the importance of cytoplasmic inheritance for PLS resistance. Resistance was highly heritable indicating that selection could be used to improve the resistance in this germplasm.

The phaeosphaeria leaf spot of maize in Brazil: evidences of a new etiological agent?

The phaeosphaeria leaf spot of maize in Brazil: evidences of a new etiological agent?

BRS Missões: nova cultivar de milho para a região Sul do Brasil

Para atender à demanda por cultivares de milho de polinização aberta, com elevado potencial produtivo, adaptadas às condições de clima temperado do Sul do Brasil, a Embrapa Trigo, em parceria com a Embrapa Milho e Sorgo, desenvolveu a cultivar BRS Missões, de ciclo precoce, com grãos do tipo amarelo dentado e de excelente potencial produtivo. Apresenta resistência moderada à ferrugem comum (Puccinia sorghi), ferrugem polysora (Puccinia polysora), mancha por exserohilum (Exserohilum turcicum) e pinta-branca (Phaeospheria maydis). É indicada para cultivo no Rio Grande do Sul, Santa Catarina e sul do Paraná.

Yield Loss Potential of Phaeosphaeria Leaf Spot of Maize Caused by Phaeosphaeria maydis in the United States.

The ability of Phaeosphaeria leaf spot (PLS) to reduce yields of maize was evaluated over two winter growing seasons in southern Florida. Nine commercial maize hybrids, varying in reaction to PLS, were planted in a split-plot design with hybrids as whole plots and inoculated versus uninoculated treatments as subplots. The most susceptible hybrid, Pioneer brand 3489, sustained a significant reduction in grain yield (11 to 13%) and 400-kernel weight (5 to 10%) in the trials. Regression analyses indicated that grain yields and 400-kernel weights were reduced 0.23 and 0.16%, respectively, for each percent increase in PLS severity at the mid-dent stage. Because most U.S. maize hybrids are resistant, and PLS develops late in the grain-filling period, its potential to cause substantial losses in the United States appears limited at this time.

Quantitative Trait Loci Conditioning Resistance to Phaeosphaeria Leaf Spot of Maize Caused by Phaeosphaeria maydis.

Phaeosphaeria leaf spot (PLS) is a potentially important disease of maize (Zea mays) that has appeared in winter breeding nurseries in southern Florida. Inbred lines related to B73 are particularly susceptible to Phaeosphaeria leaf spot, whereas inbreds related to Mo17 are highly resistant. A previous study of the inheritance of resistance to Phaeosphaeria leaf spot in the cross B73 × Mo17 found that resistance is highly heritable and controlled by mostly additive gene action at three or four loci. In this study, we used 158 recombinant inbred (RI) lines derived from the cross B73 × Mo17 to map quantitative trait loci (QTL) governing resistance. The RI lines along with the parent inbred lines and the F1 were evaluated for PLS resistance in replicated trials over two winter growing seasons in southern Florida. Using the composite interval mapping (CIM) function of PLABQTL software, five QTL on four different chromosomes were found to control PLS resistance in Mo17. In addition, the × additive interaction between two of these QTL was found to be significant. Our results are in close agreement with the previous study, where generation mean analysis was used to study the inheritance of resistance to PLS.