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Foliar damage can be observed at different stages of plant development. In the early seedling stage, leaves show irregular, oval-to-elongated lesions with characteristic, yellow-to-reddish-brown margins (Photo 24). In later stages of plant development, similar lesions can be observed in the upper leaves of infected plants, especially in those where stalk rot symptoms have already developed.
22 24 23 25 26 24 Yellow leaf blight Anamorph: Phyllosticta maydis (Teleomorph: Mycosphaerella zeae-maydis) In 1970 the disease was associated with susceptibility in Texas male sterile cultivars, and several researchers linked this disease with yield losses and increased lodging. Humid, warm weather favors disease development.
Young, diseased plants show symptoms similar to those observed in nitrogen deficient plants. In mature plants, lesions are narrow, necrotic, and parallel to the veins, although not limited by them (Photos 25, 26). In older leaves, lesions develop further and produce a characteristic blighting near the tip.
True to the name, this disease develops on leaves and sheaths. Symptoms are characteristic concentric spots that cover large areas of infected leaves and husks (Photos 27, 28).
The main damage in the humid tropics is a brownish rotting of ears, which show conspicuous, light brown, cottony mycelium with small, round, black sclerotia.
26 27 28 27 29 30 28 Leptosphaeria leaf spot Leptosphaeria michotii This disease has been reported in high, humid areas of the Himalayas. Other species of Leptosphaeria that produce different symptoms on maize leaves are known in other regions of the world.
Symptoms consist of small lesions that become large and concentric, covering large areas of the leaves (Photo 29). It is most conspicuous on lower leaves at flowering.
Phaeosphaeria leaf spot Phaeosphaeria maydis This disease is found in Brazil, Colombia, Ecuador, northern India, eastern and southern Africa, the USA, and Mexico, where Exserohilum turcicum is also prevalent. Conditions of high humidity and relatively low night temperatures favor development. Lesions appear as small pale green areas, which later become bleached and finally necrotic, surrounded by dark brown margins (Photo 30). Spots on leaves are round to slightly elongated.
29 Hyalothyridium leaf spot Anamorph: Hyalothyridium maydis (Teleomorph: Leptosphaerulina sp.) This is a foliar disease reported in Colombia, Costa Rica, Ecuador, and Mexico. The disease may cause severe losses in commercial plantings when conditions are hot and humid, with cool nights.
Lesions start in bottom leaves as small round freckles when plants are hip-high. Three weeks after flowering, lesions turn round, brown, necrotic and are 3-4 cm in diameter, showing concentric rings surrounded by chlorotic tissue (Photos 31, 32).
In Colombia a teleomorph stage Leptosphaerulina sp.
has been associated with the disease during later stages of infection.
30 31 32 31 33 32 Curvularia leaf spot Curvularia lunata, C. pallescens, and C. maculans These fungi produce small necrotic or chlorotic spots with a light colored halo (Photo 33).
Lesions are about 0.5 cm in diameter when fully developed. The disease is prevalent in hot, humid maize areas and can damage the crop significantly.
This disease, also known as cercospora leaf spot, may occur in subtropical and temperate, humid areas.
Lesions begin as small, regular, elongated brown-gray necrotic spots growing parallel to the veins (Photo 34). Occasionally, lesions may reach 3.0 x 0.3 cm.
Minimum tillage practices have been associated with an increased incidence of GLS. The disease is of concern in South America and eastern and southern Africa. Development is favored by extended periods of leaf wetness and cloudy conditions, and can result in severe leaf senescence following flowering and in poor grain fill.
34 34 35 35 36 Zonate leaf spot Gloeocercospora sorghi Zonate leaf spot is more commonly found in sorghum than maize.
The disease is characterized by small necrotic lesions that enlarge and produce large, concentric, necrotic rings (Photo 35). Lesions may be as large as 5 to 6 cm in diameter, and occur mainly on older leaves.
The spotting mainly affects maize grown in cool, humid environments.
Symptoms first appear as small, light-green-to-yellow spots on the leaves (Photos 36, 37). Lesions coalesce and produce severe blotching and necrosis of affected areas where many black, spore-producing structures know as pycnidia appear.
38 36 37 39 38 39 40 Eyespot Kabatiella zeae (syn. Aureobasidium zeae) Commercial plantings in countries with cool, moist environments may be affected by eyespot.
The disease is characterized by small (1 to 4 mm), round, translucent lesions. Tan colored centers develop, surrounded by black-to-purple rings, with a yellow halo around them, thus producing the characteristic “eyespot” (Photos 38, 39).
These symptoms are easily confused with physiological or genetic spots, which are noninfectious but widely observed in maize leaves.
The symptoms are also similar to early spotting induced by Curvularia.
This disease has not been reported to cause economic damage, but causes some damage in commercial maize plantings in hot, humid areas.
Stenocarpella macrospora is mostly an ear-rotting agent, but under appropriate climatic conditions can cause foliar damage.
Symptoms consist of necrotic lesions along the veins. These lesions resemble spotting produced by bacteria or by Exserohilum turcicum leaf blight). However, when held against the light, S. macrospora lesions exhibit a distinct narrow yellow margin not present in lesions caused by other pathogens (Photo 40). Under humid conditions, the black, spore-producing pycnidia formed in the necrotic lesions ooze spores in fine black threads and the lesion splits (Photo 41).
42 40 41 43 42 44 Pythium stalk rot Pythium aphanidermatum, Pythium spp.
Pythium species cause stalk rots, seed rots, and seedling blights. The disease is present in some hot and humid tropical and subtropical zones and in temperate regions.
Usually the basal internodes become soft, watersoaked, and dark, causing lodging. Damaged internodes commonly twist before the plants lodge.
Diseased plants can remain alive until all vascular bundles become affected (Photo 42).
Isolations in culture media are necessary to differentiate Pythium from Erwinia stalk rots.
Plants can be affected prior to flowering.
Gibberella zeae (Anamorph: Fusarium graminearum) These two species of Fusarium are responsible for stalk
rots in maize:
Fusarium moniliforme is most common in dry, warm areas. It is particularly severe if it begins just before tasseling (Photo 43).
Gibberella zeae is prevalent in cool regions. It is one of the most potentially damaging stalk-rotting agents (Photo 44).
Symptoms produced by these pathogens resemble those caused by Stenocarpella or Cephalosporium, and cannot be differentiated until spore-producing structures are observed. Wilted plants remain standing when dry, and small, dark-brown lesions develop in the lowest internodes. When infected stalks are split, the phloem appears dark brown, and there is a general conspicuous browning of tissues.
In the final stages of infection, pith is shredded and surrounding tissues become discolored.
46 43 44 47 45 46 48 Head smut Sphacelotheca reiliana Head smut can cause significant economic damage in dry, hot maize growing areas, as well as in midhill zones and under temperate conditions.
The infection is systemic: the fungus penetrates the seedlings and grows inside the plant without showing symptoms, until the tasseling and silking stages.
The most conspicuous symptoms are (a) abnormal development of the tassels, which become malformed and overgrown (Photo 45); (b) black masses of spores that develop inside individual male florets; and (c) masses of black spores in place of the normal ear, leaving the vascular bundles exposed and shredded (Photo 46).
False head smut occurs very rarely in hot, dry or humid areas. The fungus commonly infects rice flowers more than maize.
Symptoms differ from those of other smuts of maize. False head smut produces neither tassel malformation nor ear infection, as does true head smut (Sphacelotheca reiliana); only a few isolated male florets in the tassel show darkgreen masses of spores (sori; Photo 47). False head smut also differs from common smut (Ustilago maydis) in that no galls are produced.
50 47 51 48 49 52 Black bundle disease and late wilt Acremonium strictum (syn. Cephalosporium acremonium) and C. maydis Black bundle disease is caused by Cephalosporium acremonium and is widely distributed. The late wilt disease, caused by C. maydis, has been reported only in Egypt and India. Both diseases kill the plants near flowering time (Photo 48). They are most common in humid, heavy soils in hot areas. The pathogens are soil- and seed-borne.
Infected plants do not show symptoms until they reach tasseling stage and start wilting, generally beginning from the top leaves. Diseased plants produce only nubbins or ears with underdeveloped, shrunken kernels. When split, diseased stalks show brown vascular bundles starting in the underground portion of the roots (Photo 49). Similar symptoms may be observed in plants damaged by Fusarium moniliforme.
53 Anthracnose stalk rot Anamorph: Colletotrichum graminicola (Teleomorph: Glomerella graminicola) The fungus Colletotrichum graminicola causes both a stalk rot and a leaf blight. The stalk rot is found mostly in warm, humid areas throughout the world.
Infection symptoms are clearly evident as narrow, elongated dark lesions (initially brown; turning later to black) along the stem surface beginning when plants approach flowering (Photo 50). In infected plants, there is premature wilting due to the complete destruction of pith tissue, with shredded vascular bundles turning dark brown (Photo 51).
Because this and other fungi overwinter in infected maize tissues, conservation agriculture practices involving mulches reportedly increase the incidence of the disease.
(Photo courtesy of Dr. R. Carvalho) 54 50 51 55 52 53 56 Charcoal stalk rot Macrophomina phaseolina Charcoal stalk rot is most common in hot, dry environments. Incidence increases rapidly when drought and high temperatures prevail near tasseling stage.
The pathogen invades seedling roots. After flowering, initial symptoms are the abnormal drying of upper leaf tissue. When plants approach maturity, the internal parts of stems show a black discoloration and vascular bundles shred (Photo 52), mainly in lower stalk internodes. Careful examination of rind and vascular bundles reveals small, black, fungal structures known as sclerotia (Photo 53) that can overwinter and infect the next crop. The fungus may also infect kernels, blackening them completely.
Many crops can serve as hosts for this pathogen.
The disease was first reported in India, but has been found in several other countries in Asia, Africa, and the Americas. It develops in hot, humid environments.
Diseased plants dry prematurely. Splitting stalks open will show some shredding of the pith and a dark gray to black discoloration of the vascular bundles.
Abundant grayish mycelia are conspicuous in the rotten areas, confined mostly to the lower internodes above ground (Photo 54).
Unlike charcoal rot, Botryodiplodia stalk rot does not produce black pinhead-like sclerotia in the rotten areas, but it does produce abundant, gray-blackish, cottony mycelium in cavities formed in the pith of affected internodes.
58 54 59 55 56 60 Stenocarpella stalk rot Stenocarpella maydis, syn. Diplodia maydis S. macrospora, syn. D. macrospora Stalk rot is caused by S. maydis in cool, humid temperate areas, and by S. macrospora in warm, humid zones. In susceptible cultivars it causes browning of the pith of basal internodes (Photos 55, 56). Stalks are weakened and break easily during strong winds and rains. Late in the season, the most conspicuous symptom is the abundant formation of spore structures known as pycnidia on the surface of internodes where rotting has occurred.
61 57 58 62 Penicillium ear rots Penicillium spp.
Damage is most frequently caused by Penicillium oxalicum, but other species may occasionally be involved. In many instances infection follows ear damage by insects.
A conspicuous, light blue-green powder grows between kernels and on the cob surface (Photos 57, 58). Kernels with fungal growth normally become bleached and streaked.
The disease may be a serious problem when infected ears are stored at high moisture contents. Several species of Aspergillus can infect maize in the field. Aspergillus niger is the most common; it produces black, powdery masses of spores that cover both kernels and cob. In contrast, A. glaucus, A. flavus (Photo 59), and A. ochraceus normally form yellow-green masses of spores. Aspergillus parasiticus is ivy green and less common in maize.
Aspergillus flavus and A. parasiticus produce mycotoxins known as aflatoxins that are harmful to birds and mammals.
64 59 65 60 61 62 66 Fusarium and gibberella ear rots Fusarium graminearum (syn. F. roseum) (Teleomorph: Gibberella zeae) Fusarium moniliforme, syn. F. verticillioides (Teleomorph: Gibberella fujikuroi) In maize, these two species of fungi cause ear rots, stalk rots, and seedling blights. Gibberella zeae, the sexual stage of the pathogen, is most common in cool and humid areas. Ear infection begins as white mycelium moving down from the tip, which later turns reddish-pink, in infected kernels (Photo 60). The fungus produces mycotoxins—known as deoxynivalenol, zearalenone, and zearalenol—which are noxious to several animal species.
Fusarium moniliforme ear rot is likely the most common pathogen of maize ears throughout the world. In contrast to damage from G. zeae, that from F.