Maydis leaf blight (also known as southern maize leaf blight) is caused by the ascomycete fungi Bipolaris maydis and is reported from most maize growing regions of the world. Maydis leaf blight is most serious in warm and wet temperate and tropical areas, where yield losses close to 70% have been reported due to the disease. Several races of B. maydis are pathogenic to maize. Symptoms and severity of B. maydis depends on the pathogen race and host germplasm.
- Bipolaris maydis (syn. Helminthosporium maydis).
- Teliomorph: Cochliobolus hetrostrophus.
- There are 3 physiological races of B. maydis: Race T, Race O and Race C. Race T and Race C are pathogenic only to maize germplasm with cytoplasm male-sterile T and cytoplasm male-sterile C respectively.
Symptoms of Maydis leaf blight vary according to the causal race and host germplasm. Race O produces lesions that are initially small and diamond-shaped. These lesions elongate as they mature, although growth of lesions is restricted by leaf veins. Final lesions are rectangular (2-6 × 3-22 mm), restricted by leaf veins, and tan in color. Lesions caused by isolates of Race O are restricted to leaves.
Symptoms of Maydis leaf blight caused by Race T are oval and slightly larger (6-12 × 6-27 mm) than those caused by Race O. Lesion borders are usually characterized by dark, brown borders. Race T causes lesions on all above ground parts of the plant (including stems, sheaths and ears) and can also cause ear rots. Seedlings from seeds infected with Race T often wilt and die within 3 to 4 weeks.
Under severe disease pressure, usually when infection occurs prior to silking, lesions may coalesce, blighting the entire leaf. In these circumstances, sugars may be diverted from the stalk for grain filling, thus predisposing the plant to lodging.
Identity of B. maydis can be confirmed by microscopic examination of conidia that are produced within lesions and in culture. Conidia are also produced profusely within lesions when diseased leaves are incubated in a damp chamber for 24 to 48 hours. Conidia are 15-20 × 70-160 µm in dimension. Conidia are olivaceous brown, spindle-shaped, and taper to round ends. Conidia are 5 to 11 septate and are characterized by bipolar germination.
The asexual stage (conidial stage) of the fungi is the most important source of inoculum, both during the season and at the start of the season. The sexual stage of the fungi has only been documented in culture. The sexual stage is characterized by production of ascospores (typically 4 to 8) in cylindrical, hyaline asci. Ascospores are 6-7 × 130-340 µm in size, dark, and have 5 to 9 septa.
Confirmation of the physiological race can be achieved by assessing symptoms and pathogenecity on various host germplasm. Race C has only been documented in China. Race T and Race O are morphologically indistinguishable, although Race T is only pathogenic to maize where the Texas source of male sterility has been incorporated, while Race O is not. Additionally, Race T produces larger lesions than Race O and all above ground parts of the plant are susceptible to infection by Race T, including stems, sheaths and ears.
Why and where it occurs
Maydis leaf blight is distributed worldwide, although Race C is only known to occur in China.
Figure 1. Geographic distribution of Maydis leaf blight
Regions with a warm (20 to 32°C) and damp growing season are most at risk from Maydis leaf blight. Long, dry, sunny periods during the growing season are unfavorable for the disease. The fungi overwinter in infected crop debris and therefore regions where infected crop debris overwinters and where maize is cultivated continuously are at risk.
Race O pathotypes are widely distributed, although Race T pathotypes are only prevalent where varieties where the Texas source of male sterility has been incorporated.
In addition to maize, B. maydis is also known to infect sorghum and teosinthe.
The fungi overwinter as mycelium in infected crop debris that remains on the soil surface between growing seasons. At the onset of the subsequent growing season, in response to favorable temperatures and humidity, mycelia within the crop debris begin sporulating (producing condia). Conidia are then wind and rain splash disseminated to freshly planted maize in the vicinity. Conidia germinate and infect plants through stomata, giving rise to characteristic lesions within which conidia are produced, which leads to secondary disease cycles. In favorable environmental conditions, the disease cycle can be completed in as little as 72 hours.
- Mechanism of damage: Damage is caused by loss of photosynthetic leaf area, due to foliar lesions which reduce photosynthate production for grain filling. Further damage is caused by lodging, which occurs when plants divert sugars from the stalks for grain filling during severe disease pressure. Race T pathotypes are also able to infect stems, sheaths and ears, which can result in ear rot, ear drop, and lodging.
- When damage is important: Damage is most critical if infection occurs prior to silking and if weather conditions are favorable for disease development during the reproductive growth stages. If cultivating varieties with the Texas source of male sterility, infection by Race T pathotypes can lead to extensive stalk and ear rots. Seedlings grown from kernels infected with the pathogen may die four weeks following planting. Increased application of nitrogen fertilizer and increased crop density are associated with increased disease severity.
- Economic importance: Yield losses as high as 68% have been recorded due to B. maydis in Cameroon.
Maydis leaf blight is found in all tropical and temperate maize growing regions where the growing season is characterized by warm and wet conditions (Figure 1). Race C has only been documented in China.
Host resistance is the most cost-effective and practical means of disease management.
Several sources of resistance (single gene and polygene) have been recorded.
Acceptable levels of resistance are available in many tropical and temperate varieties.
Normal cytoplasm maize is resistant to Race T and Race C.
As the pathogen is only able to overwinter in infected crop debris, management of crop debris between growing seasons can help reduce the initial amount of inoculum. However, this practice will only be effective if all farmers in the vicinity coordinate their activities, as inoculum is often wind disseminated. Additionally, residue management may not be compatible with conservation farming techniques, which are more economically and environmentally sustainable.
Rotation with non-host species and reduced maize monoculture will help reduce pathogen inoculum at the onset of the growing season.
Decreasing plant density and timing planting to avoid periods favorable for the pathogen can aid in minimising yield loss.
In the absence of host resistance, fungicides can be used to provide adequate protection from Maydis leaf blight.
Fungicides should be applied when lesions are first observed and may need to be repeated during the season, depending on prevailing weather conditions.
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Contributor: Biswanath Das