2014. szeptember 30., kedd

Plant protection of maize

In the plant protection of maize, the integrated protection has to be applied. Thus, the selection of the growing area, that of the hybrid and the professional application of the agrotechnical elements (crop rotation, tillage, sowing, etc.) are important. In the case of adequate crop rotation, the most important field of the protection of maize is weed control. With crop rotation, we can successfully protect against either the animal pests (mainly the American corn rootworm) or the plant diseases (mainly Fusarium).

The diseases of maize and the protection against them

Currently, in the domestic maize production, a relatively low number of pathogens can be observed, their damages differ, primarily depends on the cropyear.

The principal maize diseases considered are the following:

·         Setosphaeria turcica (Northern Leaf Blight);
·         Sphacelotheca reiliana (Head smut);
·         Ustilago maydis (Common smut);
·         Colletotrichum graminicola (anthracnose);
·         Sclerophthora macrospora (downy mildew);
·         Nigrospora oryzae
·         Maize dwarf mosaic virus
·         Fusarium spp. (Cob and stem rot);

Northern Leaf Blight, caused by Setosphaeria turcica /anamorph Exserohilum turcicum/ (Figure 1). It is a serious fungal disease prevalent in cooler climates and tropical highlands wherever maize is grown. It is characterized by large elliptical shaped necrotic lesions that develop on the leaves due to the polyketide metabolite monocerin.

Figure 1 Setosphaeria turcica (Source: Takamiya and Sendo, 2000)

Head smut (Figure 2/a) is a disease of maize found in the major maize growing areas of Europe and the United States. The causal agent is Sphacelotheca reiliana, a fungus that attacks corn plants sporadically throughout the field, invading some plants, but leaving others unaffected. The pathogen cannot be transmitted from one plant to the other in the field and affected plants have no grain development. Even with a relatively low percentage of infection in the fields (10%), yield reduction can be significant. Infection rates up to 80% have been reported. Once the infection occurs, there are no effective treatments for reducing or eliminating the damage on affected plants.

      Figure 2/a-b Sphacelotheca reiliana and  Ustilago maydis (Source: Szőke, 2013)

Common smut of maize, caused by Ustilago maydis (Figure 2/b), is easily identified by tumor-like galls that form on actively growing host tissues and contain masses of dark, sooty teliospores. Throughout most of the world, common smut is considered to be a troublesome disease of maize. In addition to the practical significance of causing a prevalent disease and being an edible fungus, U. maydis also has been used as a model organism to study a variety of interesting biological phenomena.

Colletotrichum graminicola (Figure 3) causes anthracnose stalk rot and leaf blight of maize. C. graminicola is a major cause of stalk rot disease, one of the most economically important diseases of maize. Industry estimates are that stalk rots causes maize yield losses in the range of 6% annually. C. graminicola also causes a leaf blight that is becoming increasingly important, particularly in the tropics and subtropics. C. graminicola is among the best characterized and most tractable of the Colletotrichum fungi. It is one of very few in which sexual crosses can be made (the teleomorph is Glomerella graminicola); it is easily cultured and stored; transformation and gene disruption are routine; and pathogenicity assays are straightforward. Maize, its host, is a classical genetic model as well as an important crop plant.

 Figure 3 Colletotrichum graminicola (Source: Robertson, 2009)

Sclerophthora macrospora (Figure 4) causes systemic infection. Symptoms vary greatly with the time of infection and the degree of colonization. Plants show excessive tillering and narrow chlorotic leaves, but the most characteristic symptom, usually known as crazy top, consists of partial or complete proliferation of the tassel, which is finally transformed into a mass of leafy structures. Phyllody may also occur in the ears. Infections start from the resting oospores which are frequently formed on perennial grasses. Seed-borne mycelium can also be a source of infection. The disease usually occurs only in restricted, low-lying, inadequately drained areas or after severe floods.

Figure 4 Sclerophthora macrospora (Source: Boehm, 2000)

Nigrospora oryzae (Figure 5) causes systemic infection. Causal agent infects maize seedlings and adult plants. The lower part of pedicle turns brown and softens; plants lodge and rot. In the following vegetation period, the first symptoms of the Nigrospora Ear Rot are shredding of pedicle and lower part of shanks, black spore masses are scattered in shredded pith of the cob and on the kernel tips. Later black abundant scurf develops on cob and on reproductive buds in leaf sheaths. Under strong disease development ears remain undeveloped and lightweight. Affected kernels are dull, slightly grayish, and undeveloped.

Figure 5 Nigrospora oryzae (Source: Agroatlas, 2003)

Maize dwarf mosaic virus (Figure 6). The infected plants have, at the base of the leaves, a light green – yellow striped pattern, which develop along the leaves. All of the leaves are infected on the plant, the ear covering ones too. The light green patterns often occur around dark green islands on the leaves. At higher temperatures, the symptoms are lighter and tend to disappear, but the youngest leaves are very light green. The plant grows slowly, slightly stunted. Earlier infection may cause stalk and root rot, and the infected young plant may die. The symptoms, except for depressed growth, are very similar to other viruses or to physiological yellowing. The viruses always occur in the field sporadically, the physiological problems in smaller-bigger spots. Maize viruses often occur together, “pure” MDMV infection is extraordinarily rare. Consequently, that nearly all maize viruses can survive the winter in the Johnson grass, and the vectors transport them usually together.

Figure 6 Maize dwarf mosaic virus (Source: Agroatlas, 2003)

The most considerable damages are caused by Fusarium spp (FigureF. graminearum, F. culmorum, F. verticillioides and F. subglutinans. They can be seed-borne but, in general, survival is on plant debris. Air-borne spores are the main source of inoculum. Symptoms appear at ripening. The pathogens attack the stem from the soil through the roots or lower stem internodes. After a severe attack, the foot of the stem rots completely and the stem dies. Partly severed ears may hang down after being attacked, which can obstruct the harvesting of the maize plants and cause severe yield reduction. Depending on the year, the pathogen may cause yield losses of 6–35%, but the qualitative damage caused by the mycotoxins produced by these pathogens is a much bigger problem. These secondary metabolic products not only result in a reduction in the nutritional value of the feed, but are also absorbed by the digestive system, accumulating in the tissues and causing severe abnormalities in the digestive and sexual organs. They also have an indirect effect, since the consumption of animal products contaminated with mycotoxins represents a potential risk for humans. The most effective way of combating these pathogens is through maize resistance breeding. The mechanism of resistance to individual Fusarium species has not yet been completely clarified.

 Figure 7/a-b Fusarium ear and stalk rot (Source: Szőke, 2011)

The animal pests of maize and the protection against them

Maize stocks can be threatened by animal pests from sowing to harvest. Birds and mammals cause considerable damages only occasionally. The significance of insect pests are much greater, among them the most important ones are the following:

·         Soil insects (Agriotes spp., Melolontha spp., Tipula spp.)
·         Tanymecus dilaticollis (Maize leaf weevils)
·         Oscinella frit (Frit fly)
·         Rhopalosiphum maidis (Aphids)
·         Ostrinia nubilalis (European corn borer)
·         Helicoverpa armigera (Cotton bollworm)
·         Diabrotica virgifera virgifera (American corn rootworm)

Soil insects

The larvae of certain Elateridae (Agriotes spp., wireworms, Figure 8), Melolonthidae (Melolontha spp., white grubs, Figure 9) and Athoinae (Athous spp.) feed on roots of maize and even on the stem base. Development of wireworms takes several years; consequently, larvae of different sizes may be present in the soil when maize is planted. Development of white grubs takes 3-4 years and is generally synchronized. Damage normally only occurs from the third larval stage onwards, starting in the year after adult flight.


Figure 8 Agriotes larvae (Source: Obermeyer, 2014)

Figure 9 Melolontha larvae (Source: Božič, 2013)

Adults attack young seedlings (rarely germinating seeds) and destroy them. They feed on young leaves from the leaf margin, and most damage occurs before the 4-leaf stage (BBCH 14). Higher temperatures enhance feeding. Tanymecus dilaticollis (Figure 10) has one generation per year and overwinters as pupae in the soil.

                                           Figure 10 Tanymecus dilaticollis (Source: Slutsky, 2013) 

Figure 11 Oscinella frit larvae in a grass stem (Source: Nastić, 2013)

The pest occurs in the central, northern, and western parts of the EU. Its damage is usually sporadic, rarely attacks whole big fields. The Oscinella frit larvae (Figure 11) of the pest can destroy the productive apex of the plant, so the maize plant may die. This pest is the main damager of gardens and smaller fields; it rarely causes hard yield losses on the great fields. The rolled end of the leaves does not open itself. The youngest leaves are well etiolated and the plant growth stops. Later the whole plant dies. The symptoms can be confused with hormone-containing herbicides. Leaf-rolling is a common symptom, but the etiolating of central leaves is characteristic of the frit fly.

Maize leaf aphid (Rhopalosiphum maidis, Figure 12) feed on sorghum, maize, small grains, and other grasses. Some year's populations are numerous on small grain seedlings in October and cause reduced yields. They also are known vectors of barley yellow dwarf virus. From early July until fall, colonies of maize leaf aphids can be found on or near tassels or whorl leaves in most maize fields. Some fields may have up to 50 per cent plant infestation level, but this is extremely rare. Normal ranges are from zero to possibly 2 or 3 percent. There are no data to verify that corn leaf aphids cause barren stalks. However, it is believed that 30 to 40 percent of heavily infested stalks may be barren. Periods of dry weather seem to favor increases in aphid numbers and resulting plant damage. Dry conditions add stress to maize plants and also prevent the development of the fungal pathogens that infect and kill the aphids. Maize leaf aphids feed by sucking sap from the upper leaves and tassels. The infested tassels become covered with a sticky substance called honeydew that drips onto the leaves and silks. Pollination probably is affected by honeydew covered silks. Heavily infested leaves and tassels may wilt and turn brown. A few weeks after the initial infestation, plants will have a black, sooty appearance due to a sooty fungus that develops and thrives on the honeydew excreted by the aphids.

Figure 12 Rhopalosiphum maidis clustered on maize ear (Source: Omafra, 2009)

The European corn borer (Ostrinia nubilalis, Figure 13), which is to be found almost universally in Europe and America, is an extremely important pest from the economic point of view. Losses caused by the pest range from 250–1000 kg/ha depending on the degree of infestation, the year and the yield averages. This fact justifies protection measures in Hungary on the whole of the seed production and sweetcorn fields and on 40 % of the commercial maize sowing area. In addition to the direct damage, indirect losses are also considerable, since the injuries caused by the pest facilitate infection by Fusarium species. Maize is the preferred host plant of the pest, despite the fact that maize is indigenous in America and the corn borer in Eurasia. It is interesting to note that it was never able to spread in such great proportions on its original hosts (hemp, hops, millet and sorghum) as it has in maize.

Figure 13 Larvae of Ostrinia nubilalis (Source: Rice, 2007)

This migrant species causes serious losses throughout its range. Cobs are invaded, and developing grain is consumed. There are two to three generations a year. Helicoverpa armigera (Figure 14) overwinters as pupae in the soil. Adults appear from May until the end of October. Eggs are laid on plants at or near flowering. The principal host on which eggs are laid is maize, but other plants are also concerned, e.g. tomatoes, weeds, and, in certain areas, cotton. The feeding larvae can be seen on the surface of plants, but they are often hidden within plant organs. Bore holes may be visible, but otherwise it is necessary to cut open the plant to detect the pest. Secondary infections are common.

Figure 14 Helicoverpa armigera (Source: Szőke, 2013)

 Until recently, apart from a few pathogens, it has suffered little damage from pests, but this situation changed after the appearance of the corn rootworm in 1995, which became a major maize pest within a few years. It is estimated that around 15.000 ha were affected in 2007, with damage of economic proportions on almost 7000 ha. The yield losses caused by the pest may range from only a few per cent to as much as 70–80% (Figure 15). The more significant damages are caused by its larvae by chewing the roots, but the imago can also cause severe damage by chewing the pistils (defective fertilization). In certain cropyears, the corn borer (decline of stem strength) and the cotton bollworm (chewing of the grain) can cause considerable damages. Against the American corn rootworm and the soil pests – in addition to crop rotation –, we can protect by chemical soil disinfection the most successfully. In fodder maize, in the case of intensive colonization, we can protect against the imago of the American corn rootworm with insecticides.


Figure 15. Damaged of Diabrotica virgifera virgifera larvae and imago (Source: Szőke, 2012)

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