Maize is subject to attack by a number of pests and diseases, mainly in insect pests (stalk borers and armyworms) and soil pests (wireworms and rootworms).
Stalk borers are present wherever maize is grown; damage to the crop is caused by the caterpillars which feed on the stalk, whorl or the ear of the maize. Some idea of the amount of damage caused can be gauged from an experiment in east Africa, in which yields of maize were increased 2 fold as a result of insecticidal treatment. The European corn-borer Ostrinia nubilalis has a world wide distribution and causes great economic damage. In the Americas, main borers are in genera Zeadiatraea, Diatraea and Elasmopalpus; in Africa, species of Chilo, Sesamia, and Busseola, and in Southeast Asia Chilo, Sesamia and Ostrinia furnicalis. In the armyworms, the fall armyworm Spodoptera frugiperda is the most important budworm in the tropical Americas, and species of Pseudaletia and Mithimna are important in the Americas and Southeast Asia respectively. The corn earworm in the genus Heliothis is another very important pest recorded in North and South America, Europe, India, the Far East, Africa and Australia. In many regions, especially highlands of South America and the Far East, it is considered to be the most important insect pest. Wireworms, which are also serious pests of maize, have a more limited distribution than ear and stalk borers. Wireworms and rootworms in the genus Diabrotica occur mainly in the more northern latitudes of Europe and in the USA. Injury is caused by the feeding of the larval stage on parts of the plant below the soil surface, especially the seed and the stem.
Stored-grain insects can also produce great damage. Species in the genera Ephestia, Rhyzoperta, Sitophilus, Tribolium, Sitotroga, Cathartus, Dinoderus and Trogoderma have the widest distribution and cause major damage in stored grains. Maize rusts (especially Puccinia sorghi and P polysora) and leaf blights (especially Helminthosporium turcicum and H maydis) are the most damaging diseases. Rust due to Puccinia sorghi is found in the USA and is common throughout highlands of subtropical areas of Latin America and Africa. Previously regarded as of minor importance, P sorghi rust became increasingly prevalent in the USA during the 1950s when open pollinated varieties of maize were replaced by P sorghi susceptible hybrids. The rust caused by P polysora is of some importance in southern USA, but is more common in the tropical areas of the Americas. Since 1948, it has spread with remarkable speed. In 1949 it was found in Sierra Leone, and in 1950 in the Ivory Coast, Ghana and Southern Nigeria. Then it moved on to the Cameroons, to east Africa and far as Zululand. A few months later it was spreading through the scattered islands of the Indian Ocean, reaching Mauritius, Reunion, Madagascar and the Seychelles. Finally from these islands it invaded North Borneo, Thailand, Malaya, and the Philippines. The effect of the maize varieties grown in Africa and Asia was devastating, with up to 70% crop losses. Thus it was and still is a fearful threat to all those Africans and Asians dependent on maize for much of their food. Helminthosporium turcicum leaf blight is common in cool, moist areas, and may decrease yields mainly when infection occurs at the silking stage of the plant. H maydis race T caused great losses on maize production of USA during the early 1970s due to susceptible germplasm included in commercial hybrids through the incorporation of Texas male-sterile cytoplasm source. Probably this has been one of the most devastating maize diseases ever known.
Other economically important maize diseases are stalk rots and ear rots. Stalk rots cause wilting of the plants before or after pollination thus decreasing their yield. Several ear-rotting fungi are known to affect the yield and quality of grain produced. Several mycotoxins have been identified produced by fungi, mainly in the genus Aspergillus and Fusarium which affect birds and mammals fed with infected kernels. Of major concern are the downy mildews of maize caused by several species in the genera Sclerospora, Sclerophthora and Peronosclerospora. These diseases were originally reported as limiting factors in maize production in Southeast Asian countries, and later the disease spread to African countries where maize and sorghum are cultivated. Since 1964 the disease was reported in southern Texas and northeast Mexico and has expanded rapidly in humid tropics of the Americas, probably through contaminated seed. Diseases caused by viruses (such as Maize Streak transmitted by leafhoppers of the genus Cicadulina in countries of tropical subequatorial Africa, Rayado Fino transmitted by the leafhopper Dalbulus maidis in Latin America and southern USA, Maize Dwarf Mosaic Virus transmitted either through infectious sap or the aphid Rhopalosiphum maidis) are creating concern in maize growers around the world. Spiroplasma-caused diseases like Corn Stunt transmitted mainly by the leafhopper D maidis is important in tropical Latin America, the Caribbean and Southern USA.