A 3-year study on the epidemiology of sorghum anthracnose (Colletotrichum sublineolum) and leaf blight (Exserohilum turcicum) was conducted at Alupe in western Kenya from 1994 to 1996. A nonlinear logistic model was used to summarize 72 anthracnose and 108 leaf blight disease progress curves from different planting dates and cultivars. Effects of planting date and cultivar on disease development were compared based on estimates of the rate (), the absolute rate (), inflection point () and upper asymptote (␥), disease severity at milk stage (v 95), and time taken to reach a disease severity of 2% (t 2 , an estimate of time when disease is first observed). Leaf blight epidemics always started earlier than those of anthracnose, but exhibited lower disease severity at crop maturity. Effects of planting date and cultivar on  varied between years. Delaying planting reduced time to disease onset (i.e. and t 2) and increased absolute rate of progress , resulting in maximum severity at crop 'milk stage' and maturity (v 95 and ␥, respectively). Resistant cultivars had highest and t 2 but lowest , v 95 and ␥ values for both diseases. Delaying planting affected anthracnose progress more on the resistant cultivars, in contrast with leaf blight for which progress on susceptible cultivars was more affected. The implications of these findings for disease management are discussed, and recommendations made to improve disease screening methodologies.
To assess the prevalence and severity of sorghum diseases in western Kenya, a 2-year survey was conducted (July 1995 and 1996), in 91 and 109 farmers' fields, respectively. Fields were generally <0.5 ha and production environment ranged from warm-humid to warm-semi-arid. Fourteen foliar and six panicle diseases were observed, with limited variation in disease prevalence and severity between the 2 years. The most common foliar diseases observed were (in decreasing order of prevalence) oval leaf spot (Ramulispora sorghicola), rust (Puccinia purpurea), ladder leaf spot (Cercospora fusimaculans), zonate leaf spot (Gloeocercospora sorghi), gray leaf spot (Cercospora sorghi), leaf blight (Exserohilum turcicum), and anthracnose (Colletotrichum sublineolum); with prevalence ranging from 95 to 97% of fields for oval leaf spot, and 44 to 65% of fields for anthracnose. Head smut (Sporisorium reilianum), was observed in 73 to 75% of fields, covered kernel smut (S. sorghi) 42 to 43% of fields, and loose smut (S. cruenta) 14 to 24% of fields. Head smut incidence was >25% in 3% of fields surveyed. Grain yield reduction from smut diseases alone was estimated to be 5%. Out of eight probability distribution functions compared, the double Gaussian model best described the frequency of disease severity levels for most diseases. Based on the best-fitting model, the proportion of fields with disease severity level thought to cause yield loss (severity rating >5 on a 1 to 9 scale, where 1 = no disease) was calculated as 26.6% for oval leaf spot, 15.3% for rust, 14.8% for anthracnose, 4.8% for ladder leaf spot, and 1.5% for leaf blight. The production environment influenced the prevalence of disease severity. Severe anthracnose, leaf blight, and ladder leaf spot were confined to fields in the humid LM1 and LM2 agro-ecological zones, rust was ubiquitous, and severe gray leaf spot was more prevalent in the dryer LM4 zone.
Field studies were conducted at Alupe in western Kenya in 1995 and 1996 to evaluate the efficacy of crop and species mixtures for the management of sorghum anthracnose (caused by Colletotrichum sublineolum) and leaf blight (caused by Exserohilum turcicum). The progress of these diseases developing simultaneously on a susceptible sorghum cultivar planted in inter- or intra-row mixtures of varying proportions with either maize or resistant sorghum was monitored. The effects of host type and mixture patterns on disease progress were compared by parameter estimates derived from fitted Lotka-Volterra competition equations and nonlinear logistic models. Competition coefficients were not significant and their confidence intervals included zero in most cases, suggesting that interactions between C. sublineolum and E. turcicum did not occur. Mixtures of the susceptible sorghum with either the nonhost maize or the resistant sorghum delayed the time when disease is first observed and reduced the rate of disease progress and carrying capacity for both anthracnose and leaf blight, with a more pronounced effect on the latter disease. The lower efficacy of mixtures in reducing anthracnose was attributed to an aggregated spatial pattern, coupled with higher rates of progress for this disease. Intra-row mixtures were more efficient than inter-row mixtures in reducing disease development in all years. The implications of these observations for the management of sorghum diseases under small-scale farming systems are discussed.
Southern corn rust, caused by Pucclnia polysora Underw. causes severe yield losses on maize (Zea mays L.); especially on late planted maize in the South. Near‐isogemc crosses were used to determine the extent of yield losses caused by this disease. Susceptible hybrids, in three biweekly plantings (beginning about 25 May) in 1976 and 1978, showed average yield reductions of 4, 23, and 45%. Reduced kernel weight accounted for some but not all of the loss in yield.
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