Use of Disease Assessment Methods in Predicting Yield Loss Due to Northern Leaf Blight of Maize

Different severities of northern leaf blight were induced by inoculating maize plants once at GS 4 and 5, three time at GS 4,5 and 6, allowing natural disease development, and by applying fungicides to deter disease development on cultivars with different levels of resistance to Exserohilum turcicum . Percentage leaf area affected by northern leaf blight on the whole plant, ear, first, and second leaf above the ear leaf were assessed weekly for a total of six times at GS 8,9.0,9.1,9.2,9.3, and 9.4. Data obtained from the ear leaf, first leaf above the ear leaf, and second leaf above the ear leaf were used to develop a one-three-two weighted scale to account for leaf position and time of infection for the three leaves assessed and were related to yield by regression analysis. The average disease severity estimated on the ear leaf was significantly (P ≤ 0.001) correlated with severities estimated using other disease assessment methods and loss in grain yield. Overall, AUDPC yield-loss models using percentage leaf area affected on the ear leaf gave the best relationship (Y= 5835-135 AUDPC, R2 = 0.42, P ≤ 0.001). Further analysis using leaves of various positions on the plant did not improve the yield-loss models. Critical point models, using percentage leaf area affected at GS 9.1 on A619xA632 also gave good fit (R2 = 0.53)

Use of Disease Assessment Methods in Predicting Yield Loss Due to Northern Leaf Blight of Maize

Different severities of northern leaf blight were induced by inoculating maize plants once at GS 4 and 5, three time at GS 4,5 and 6, allowing natural disease development, and by applying fungicides to deter disease development on cultivars with different levels of resistance to Exserohilum turcicum . Percentage leaf area affected by northern leaf blight on the whole plant, ear, first, and second leaf above the ear leaf were assessed weekly for a total of six times at GS 8,9.0,9.1,9.2,9.3, and 9.4. Data obtained from the ear leaf, first leaf above the ear leaf, and second leaf above the ear leaf were used to develop a one-three-two weighted scale to account for leaf position and time of infection for the three leaves assessed and were related to yield by regression analysis. The average disease severity estimated on the ear leaf was significantly (P 64 0.001) correlated with severities estimated using other disease assessment methods and loss in grain yield. Overall, AUDPC yield-loss models using percentage leaf area affected on the ear leaf gave the best relationship (Y= 5835-135 AUDPC, R2 = 0.42, P 64 0.001). Further analysis using leaves of various positions on the plant did not improve the yield-loss models. Critical point models, using percentage leaf area affected at GS 9.1 on A619xA632 also gave good fit (R2 = 0.53)

Inheritance of race-nonspecific resistance to Exserohilum turcicum in maize synthetic population OhS10

Progeny tests of resistant lines derived from maize (Zea mays L.) synthetic population OhS10 were conducted at one location during two years (1993-94) in Ohio (Guerne) and at two locations during one year (1993) in Uganda (Kabanyolo and Namulonge) to examine the inheritance of resistance to Exserohilum turcicum(Pass.), causal agent of northern leaf blight (NLB). Individual plants in single ear derived selfed progenies (S1 and S1:2) were inoculated in a controlled fashion (Guerne, Namulonge) or naturally infected (Kabanyolo). Host responses to infection were characterized by determining area under the disease progress curve (AUDPC), calculated from percent leaf area affected (PLAA) and lesion number counts, and lesion size. The majority of S1 and S1:2 progenies had few lesions, low rates of disease development, and high levels of resistance, indicating that multiple factors govern expression of resistance. Of the progenies tested, S20-4, S30-5, S30-6 and S30-7 exhibited consistently high levels of resistance to NLB.Des tests de descendance des lignees resistantes obtenues a partir d'une population synthetique de mais OhS10 ont ete executes a un endroit pendant deux annees (1993-1994) Ohio (Guerne) et a deux endroits pendant une annee (1993) en Ouganda (Kabanyolo et Namulonge) pour examiner la transmission de la resistance a Exserohilum turcicum (Pass.), qui cause le rouille des feuilles (qui sevit au nord) (NLB). Les plantes individuelles des progenies obtenues par descendance autofecondee issue d'un epis unique (S1 et S1:2) etaient inoculees de facon controee (Guerne, Namulonge) ou infectees naturellement (Kabanyolo). Les reponses des infections etaient caracterisees par la determination de la surface en dessous de la courbe de progres de la maladie (AUDPC), qui etaient calculee a partir du pourcentage de surface de la feuille affectee (PLAA) et le nombre des lesions et la taille de lesions. La majoritedes descendants S1 et S1:2 avaient peu de lesions, un developpement ralentie de la maladie et un niveau de resistance eleve, ce qui indique que plusieurs paramtres influencent l'expression de la resistance. S20-4, S30-5, S30-6 et S30-7 montrent un niveau de resistance eleve contre NLB

The effect of potato variety mixtures on epidemics of late blight, in relation to plot size and level of resistance

Potatoes of a number of varieties of contrasting levels of resistance were planted in pure or mixed stands in four experiments over 3 years. Three experiments compared the late blight severity and progress in mixtures with that in pure stands. Disease on susceptible or moderately resistant varieties typical of those in commercial use was similar in mixtures and pure stands. In 2 of 3 years, there were slight reductions on cv. Sante, which is moderately susceptible, in mixture with cv. Cara, which is moderately resistant. Cara was unaffected by this mixture. Mixtures of an immune or near-immune partner with Cara or Sante substantially reduced disease on the latter. The effect of the size of plots of individual varieties or mixtures on blight severity was compared in two experiments. Larger plots had a greater area under the disease progress curve, but the average rate of disease progress was greater in smaller plots; this may be because most disease progress took place later, under more favourable conditions, in the smaller plots. In one experiment, two planting densities were used. Density had no effect on disease and did not interact with mixture effects. The overall conclusion is that, while mixtures of potato varieties may be desirable for other reasons, they do not offer any improvement on the average of the disease resistance of the components