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«4th edition The CIMMYT Maize Program CIMMYT® (is an internationally funded, not-for- profit organization that conducts research and ...»

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Maize Diseases:

A Guide for Field

Identification

4th edition

The CIMMYT Maize Program

CIMMYT® (www.cimmyt.org) is an internationally funded, not-for-

profit organization that conducts research and training related to

maize and wheat throughout the developing world. Drawing on

strong science and effective partnerships, CIMMYT works to create,

share, and use knowledge and technology to increase food security,

improve the productivity and profitability of farming systems, and sustain natural resources. Financial support for CIMMYT’s work comes from many sources, including the members of the Consultative Group on International Agricultural Research (CGIAR) (www.cgiar.org), national governments, foundations, development banks, and other public and private agencies.

© International Maize and Wheat Improvement Center (CIMMYT)

2004. All rights reserved. The designations employed in the presentation of materials in this publication do not imply the expression of any opinion whatsoever on the part of CIMMYT or its contributory organizations concerning the legal status of any country, territory, city, or area, or of its authorities, or concerning the delimitation of its frontiers or boundaries. CIMMYT encourages fair use of this material. Proper citation is requested.

Correct citation: The CIMMYT Maize Program. 2004.

Maize Diseases: A Guide for Field Identification. 4th edition.

Mexico, D.F.: CIMMYT.

Abstract: Intended for field use by agricultural technicians and maize farmers, this pocket-size manual carries descriptions and color photographs for more than 50 fungal, bacterial, viral, and mollicute diseases that affect the maize crop worldwide, with basic information on pathogens and symptoms. A diagnostic key facilitates quick identification of diseases and their effects. In this fourth edition, nomenclature has been updated, new diseases and information added, and improved photographs included.

ISBN: 970-648-109-5 AGROVOC descriptors: Plant diseases; Zea mays; Bacterial diseases; Viruses; Fungi disease; Mildews; Leaf area; Fusarium; Smuts;

Rots; Penicillium; Charcoal; Nigrospora; Cephalosporium; Kernels;

Botryodiplodia; Dwarfism; Necrosis AGRIS category codes: H20 Plant Diseases; F01 Crop Husbandry Dewey decimal classification: 633.15 Printed in Mexico.

Contents Introduction

Diseases caused by fungi: Foliar diseases Brown spot

Downy mildews

Common rust

Polysora rust

Tropical rust

Borde blanco, Vertical banded blight

Tar spot complex

Turcicum leaf blight

Maydis leaf blight

Carbonum leaf spot

Anthracnose leaf blight

Yellow leaf blight

Banded leaf and sheath blight

Leptosphaeria leaf spot

Phaeosphaeria leaf spot

Hyalothyridium leaf spot

Curvularia leaf spot

Gray leaf spot

Zonate leaf spot

Septoria leaf blotch

Eyespot

Macrospora leaf stripe

Diseases caused by fungi: Stalk rots and smuts Pythium stalk rot

Fusarium and gibberella stalk rots

Head smut

False head smut

Black bundle disease and late wilt

Anthracnose stalk rot

Charcoal stalk rot

Botryodiplodia stalk rot

Stenocarpella stalk rot

Contents Diseases caused by fungi: Ear rots Penicillium ear rots

Aspergillus ear rots

Fusarium and gibberella ear rots

Ergot, horse’s tooth

Charcoal ear rot

Nigrospora ear rot

Gray ear rot

Common smut

Botryodiplodia or black kernel rot

Cephalosporium kernel rot

Hormodendrum ear rot

Stenocarpella ear rot

Diseases caused by bacteria Bacterial stalk rot

Stewart’s wilt

Bacterial leaf stripe

Diseases caused by viruses and mollicutes Maize chlorotic dwarf virus (MCDV)

Maize chlorotic mottle virus (MCMV)

Maize dwarf mosaic virus (MDMV)

Sugarcane mosaic virus (SCMV)

Maize lethal necrosis (MLN)

Maize mosaic virus I (MMV)

Maize stripe virus (M StV)

Maize streak virus (MSV)

Maize rough dwarf virus (MRDV)

Maize fine stripe virus

Maize bushy stunt (MBS)

Corn stunt

Diagnostic key

Introduction This popular booklet, already in its fourth edition, is designed as a quick guide for identifying maize diseases.

Based on previous editions produced by CIMMYT maize pathologist Carlos De Leon, the new work has been updated by contributions from CIMMYT maize pathologist Dan Jeffers. It is intended for field use by agricultural technicians and maize producers, and the taxonomic short forms of the various pathogens are deemed to be appropriate and adequate. For fungal pathogens, both the sexual (teleomorph) and asexual (anamorph) names of the fungi are often included. Several modifications in nomenclature have been made since the last edition.

Common names of the diseases are designated mostly following the nomenclature described by A.J. Ullstrup (1985.

Plant Disease 69:658-659). The text comprises a brief description of common maize diseases, their causal agents, and their symptoms. Additional diseases and new information have been included, based on advances in science since the previous edition. There are numerous color photographs of diseased plants. A diagnostic key is included as an appendix. The text is divided according to the four causal agents for maize diseases: fungi, bacteria, viruses, and mollicutes. Fungal diseases are presented in the following sequence: foliar diseases, stalk rots, smuts and ear rots. Most diseases covered are economically significant or have the potential to become so.





We greatly appreciate the editorial assistance of CIMMYT science writer, Mike Listman, and the production and design skills of CIMMYT designer, Wenceslao Almazán, in producing this new edition.

–  –  –

The disease normally occurs in areas of high rainfall and high mean temperatures. It attacks leaves, leaf sheaths, stalks, and sometimes outer husks.

The first noticeable symptoms develop on leaf blades and consist of small chlorotic spots, arranged as alternate bands of diseased and healthy tissue (Photo 1). Spots on the mid-ribs are circular and dark brown, while lesions on the laminae continue as chlorotic spots. Nodes and internodes also show brown lesions. In severe infections, these may coalesce and induce stalk rotting and lodging (Photo 2).

2 1 2 3 3 5 4 4 Downy mildews Several species of the genera Peronosclerospora, Sclerospora, and Sclerophthora are responsible for

downy mildews:

Crazy top downy mildew Sclerophthora macrospora Brown stripe downy mildew Sclerophthora rayssiae var. zeae (Photo 3) Green ear disease Sclerospora graminicola Java downy mildew Peronosclerospora maydis (Photo 4) Philippine downy mildew Peronosclerospora philippinensis (Photo 5) Sugarcane downy mildew Peronosclerospora sacchari (Photo 6) Sorghum downy mildew Peronosclerospora sorghi (Photos 7, 8, 9) 5 These diseases are of serious concern to maize producers in several countries of Asia, Africa, and throughout the Americas. Symptom expression is greatly affected by plant age, pathogen species, and environment. Usually, there is chlorotic striping or partial symptoms in leaves and leaf sheaths, along with dwarfing. Downy mildew becomes conspicuous after development of a downy growth on or under leaf surfaces. This condition is the result of conidia formation, which commonly occurs in the early morning.

The diseases are most prevalent in warm, humid regions. Some species causing downy mildew also induce tassel malformations, blocking pollen production and ear formation. Leaves may be narrow, thick, and abnormally erect.

6 6 7 8 9 7 10 11 8 Maize rusts The three leaf rusts on maize are common rust, polysora rust, and tropical rust.

Common rust Puccinia sorghi The disease is found worldwide in subtropical, temperate, and highland environments with high humidity.

Common rust is most conspicuous when plants approach tasseling. It may be recognized by small, elongate, powdery pustules over both surfaces of the leaves (Photo 10). Pustules are dark brown in early stages of infection; later, the epidermis is ruptured and the lesions turn black as the plant matures.

Plants of the alternate host (Oxalis spp.) are frequently infected with light orange colored pustules (Photo 11). This is simply another stage of the same fungus.

9Polysora rustPuccinia polysora

Pustules are smaller, lighter in color (light orange), and more circular (Photo 12) than those produced by P. sorghi. They are also present on both leaf surfaces, but the epidermis remains intact longer than it does in P. sorghi-infected leaves. Pustules turn dark brown as plants approach maturity. No alternate host of the fungus is known. Polysora rust (or southern rust) is common in hot and humid lowland tropical conditions.

Tropical rust Physopella zeae Outbreaks of this rust are sporadic and confined to the American tropics.

Pustules vary in shape from round to oval. They are small and found beneath the epidermis. At the center of the pustule the lesion appears white to pale yellow and an opening develops (Photo 13). The pustule is sometimes black rimmed, but its center remains light.

No alternate host of the fungus is known.

10 12 13 11 14 15 12 Borde blanco, Vertical banded blight Marasmiellus paspali var. americanus, M. paspali var. paspali, M. paspali sensu lato Symptoms on the foliage are very similar to those produced by banded leaf and sheath blight (BLSB;

see page 27). The disease has been reported on maize growing in hot, humid, lowland tropical areas, where it causes no economic damage.

Elongated, concentric bicolored lesions start developing on margins of the leaves (Photo 14) around flowering time. At this stage, there are no apparent symptoms of fungal growth on the lesions.

Later in the season, small fungal fructifications resembling sclerotia of the causal agent of BLSB will develop. However, on closer inspection these structures are agaricoid fructifications (basidiomata) of the fungus (Photo 15). High humidity and rainfall favor the development of these ephemeral structures.

The three above-mentioned species have been reported on maize in tropical American countries, East and Southeast Asia, and West African countries, respectively.

–  –  –

The disease occurs in relatively cool, humid areas in the tropics, similar to the conditions where turcicum leaf blight is prevalent. Characteristic black, raised, shiny spots are initially produced (Photo 16). Infected foliar tissue will become necrotic and die.

In several countries in the Americas, a second pathogen, Monographella maydis, has been associated with Phyllachora maydis as part of the “tar spot complex.” This association results in the development of necrotic tissue around the tar spot (Photo 17). These necrotic lesions may coalesce, causing a complete burning of the foliage. Lesions caused by M. maydis alone are round and 5-6 cm in diameter (Photo 18).

Lesions of both pathogens involved in the tar spot complex start developing in the lower leaves before flowering time. In favorable conditions, the infection spreads to the younger leaves. Affected ears are light in weight with loose kernels. Many kernels at the tip of the ear will show premature germination while still on the cob (Photo 19).

14 16 17 19 18 15 20 16 Turcicum leaf blight Teleomorph: Setosphaeria turcica (syn. Trichometasphaeria turcica) (Anamorph: Exserohilum turcicum, syn.

Helminthosporium turcicum) An early symptom is the easily recognized, slightly oval, water-soaked, small spots produced on the leaves. These grow into elongated, spindle-shaped necrotic lesions (Photo 20). They may appear first on lower leaves and increase in number as the plant develops, and can lead to complete burning of the foliage.

Turcicum leaf blight (or northern leaf blight) occurs worldwide and particularly in areas where high humidity and moderate temperatures prevail during the growing season. When infection occurs prior to and at silking and conditions are optimum, it may cause significant economic damage.

–  –  –

Young lesions are small and diamond shaped. As they mature, they elongate. Growth is limited by adjacent veins, so final lesion shape is rectangular and 2 to 3 cm long. Lesions may coalesce, producing a complete burning of large areas of the leaves (Photo 21).

The symptoms described above correspond to the “O” strain of the fungus. In the early 1970s the “T” strain caused severe damage to maize cultivars in which the Texas source of male sterility had been incorporated. Lesions produced by the T strain (Photo 22) are oval and larger than those produced by the O strain. A major difference is that the T strain affects husks and leaf sheaths, while the O strain normally does not.

Maydis leaf blight (or southern maize leaf blight) is prevalent in hot, humid, maize-growing areas. The fungus requires slightly higher temperatures for infection than E. turcicum; however, both species are often found on the same plant.

18 21 22 19 23 20 Carbonum leaf spot Teleomorph: Cochliobolus carbonum (Anamorph: Bipolaris zeicola, syn. Helminthosporium carbonum) This disease is most common in very moist areas with moderate temperatures.

Different symptoms are produced on the leaves by the five known races of the fungus. Race 1 produces oval, zonate, brownish lesions on all parts of the plants including the ears, which rot and turn black.

Race 2 produces brown, slender, elongated lesions, mostly in the lower leaves (Photo 23), and can also produce ear rot. Race 3 produces narrow, grayish lesions with a chlorotic border. Race 4 produces lesions similar to those from Race 2, but they frequently show concentric patterns. Race 5 produces only small necrotic flecks on immature leaves.

Ear rot symptoms produced by Races 1 and 2 are very similar.

–  –  –

The disease is present in warm, humid environments worldwide, with a foliar disease phase and a stalkrotting phase.

The foliar infection phase of the fungus is not reported to be of economic importance in maize. The most severe damage is caused by the stalk rot phase.



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