Leaf blight of maize
Economically relevant diseases in maize occur more and more in all warmer maize growing areas in Germany since the middle of the nineties.
The infestation rate depends on weather conditions and is different every year.
Warm humid weather in early summer establishes favourable conditions for sever infestations.
Prolonged dry periods in summer suggest low infestation rates.
Until the middle oft the 90s leaf blight of maize was considered to be a problem of warm humid maize growing areas as in our neighbouring countries in the south and overseas. Despite the fact that some of these pathogens are also found north of the Alps since a long time, the first important economic damage however was noticed in the Upper Rhine Plain in 1995. Since then leaf symptoms have been observed more and more in all warmer German maize growing areas. Within these areas the regional infestation rate is very variable. And also infection pressure is considerably different from year to year. Warm and humid weather in early summer sets favourable conditions for an epidemic. With a long period of dry weather in summer a low infestation pressure can be expected.
Setosphaeria turcica/Exserohilum turcicum
There is a range of fungi that can cause leaf diseases in maize. The fungus, causing by far the greatest damage in our climate, is called Setosphaeria turcica as teleomorph (sexual reproduction state). In the past the anamorph (production of asexual spores called conidia) of the pathogen was named Helminthosporium turcicum. For this reason the disease was formerly known as Helminthosporium-leaf blight. Today the conidia stage is classified as Exserohilum turcicum.
The pathogen is spread worldwide. In the USA it occurs mainly in the northern Corn Belt. Therefore this pattern of damage is known as “Northern corn leaf blight” there, and as long as no sufficiently resistant varieties were available, the disease arose to a highly damaging epidemic in warm and humid years. In Austria (Styria) yield losses were assessed in susceptible varieties. In years with a warm humid growing season (as in 2002) higher infestation rates have been recorded. In the region of Inn and Danube, which were the most affected areas, we estimated that yield losses stayed below 10 percent. Only highly susceptible varieties, which are not recommended though, show losses of over 30 percent.
Starting point of the leaf blight disease are maize residues remaining on the soil surface. On this substance conidia and chlamydospores (asexually produced spores from mycelium) are formed which are distributed to the lower maize leaves by rain splashes. Temperatures between 24°C to 30°C and humid leaves during a period of 10 hours are optimal requirements for the spores to germinate and to penetrate the leaf tissue. With favourable weather conditions initial infections take place beginning from the 8 leaf stage. After 10 to 14 days already new spores (conidia) are released from the underside of the leaves and are distributed by wind to neighbouring fields. If the weather stays warm and humid for a long time during spring and summer, several cycles of infection can develop and Setosphaeria-leaf blight of maize can spread out. Weather conditions with alternating high day-time and low night-time temperatures, and thus high air humidity persisting over a longer time period as dew, fog or mist, are favourable for infections.The life cycle of Setosphaeria turcica is shown in fig. 3.
First symptoms on maize plants appear on the lower leaves. Spots that occur later, caused by spores distributed by wind, show on upper leaves. At the beginning of the infestation small, longish, watery stains arise which can grow into elongated bands of grey-green to light brown lesions. The patches converge and can reach a size of up to 20 cm long and 5 cm wide. Thus big parts of the leaf blade can die back but rarely the whole leaf does. Similar patches can also be found on maize husks. In Central Europe, except maize, no further host plants for Setosphaeria turcica are known of.
Distinct patches of infection on maize are rarely to be observed in our region before inflorescence emergence. Generally speaking the later the infection starts the lower are the yield losses. According to Austrian research an infestation before or during flowering with optimal conditions for the fungus in highly susceptible varieties leads to yield losses of up to 60%. If the infection takes place 5-6 weeks after flowering yield reduction is marginal. However infested plant residues are the origin of infection for following years.
As already mentioned several times the weather in spring and summer has a primal influence on the infection spreading. High air humidity of over 95% and temperatures of 20-30°C are ideal conditions for secondary infections. But also irrigation of maize crops, as occasionally used in the dry year 2003, enhances infections developing.
The less leaves and stalks remain on the soil surface in spring, the less spores are available to start initial infections. An essential measure in endangered areas is the working in of maize residues into the soil. Thus the basis for the pathogen is removed and the infection cycle is interrupted. To speed up decomposition of straw a thorough crushing (for example mulching) before working it into the soil is important. This measure is all the more promising the more farmers in the infested area participate. The point is to cut off the chain of infection between not decomposed residues and young maize plants.
Plants that are stressed are infested more easily by fungal diseases. On headlands or patches that receive more traffic per unit of area, when spreading manure, an earlier and heavier infestation is found. The reason for this is soil compaction in these areas and thus maize plants are weaker. In addition to soil compaction also other factors, which interfere with speedy development of maize plants (over- or undersupply with nutrients, damages caused by herbicides, stagnant water etc.), contribute to higher infestation rates of leaf diseases.
The variety has the greatest influence on infections taking place. Early maturing varieties tend to be more susceptible than late maturing ones. Indeed tolerant varieties often are not free from infestation but diseases occur considerably later. By that time, usually, natural maturation has already started. That is why the infestation doesn’t affect yield results anymore. Official services provide classification lists of varieties for infested areas. The Bavarian list of varieties can be found in the internet at www.LfL.bayern.de or received via Boards of Agriculture.
Fungicide experiments are conducted in Bavaria since 2002. Different products are applied, to varieties of different susceptibility, before flowering (height of plants about 1 m) and/or at flowering, a short time after flowering respectively. The experiments are realised on plots of 60 sqm with four replications. During the experiments it was made sure that no losses due to driving while spraying were caused. In the field this aim could hardly be achieved. As shown in figure 1 and 2 yield could not be increased in the fungicide experiments, neither when spraying before flowering nor at flowering. Thus it shows that even in a year with high infection pressure, as in 2002, fungicide applications are not cost-effective because of the extra efforts necessary (high-clearance tractor and fungicide costs). Consequently this method is not adequate for solving problems in the field within a short time. Apart from that no fungicides are registered for this indication.
Therefore variety choice is most relevant in controlling leaf blight of maize. But if the preventive measures mentioned above are not followed consistently the variety alone can not save the day.
Leaf diseases of little importance
Cochliobolus carbonum (formerlly known as Helminthosporum carbonum); this pathogen causes, in contrast to Setosphaeria turcica, much smaller, light green or yellow round patches, later up to maximal 3 cm elongate brown patches. The patches are surrounded by a dark seam too and partly show concentric zones. This disease is often found in Central Europe, but cost-effective damages have not been caused yet.
The eye spot disease is caused by the fungus Kabatiella zeae. Small, only a few millimetres big, round spots appear on leaves. The brownish red centre is surrounded by a considerably larger yellow halo. This disease appears mainly in monocultures and in crops with minimal cultivation. Under these terms infestations developed in France. Proper crushing and ploughing under of infested plant residues avoids economically relevant yield losses due to this fungal disease.
Maize rust (Puccinia sorghi) is widespread especially in tropical and subtropical areas. In Central and Southern Europe the disease mostly occurs in individual years and in restricted areas (as in Austria 1995). In midsummer 1 to 3 mm big yellow pustules develop on upper and lower surfaces of leaves which rupture shortly after and release brown spores. The major control measure for this maize disease also is ploughing under of straw.
Maize is threatened by several fungal diseases. Whether they cause cost effective damage primarily depends on the annual weather trend. Even with favourable weather conditions for the fungi yield losses can be avoided by growing less susceptible varieties along with appropriate cultivation measures.