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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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Mal de Rio Cuarto Virus in central Argentina and MRDV in northern China have been reported as seriously affecting maize production.

104 99 100 105 101 102 106 Maize fine stripe virus (Maize rayado fino virus, or MRFV) “Rayado fino” or fine stripe, is caused by a virus, transmitted by the leafhopper Dalbulus maidis. The vector will transmit the virus for most of its life after feeding on an infected plant. Dalbulus maidis is also a vector of the corn stunt spiroplasma and maize bushy stunt phytoplasma. This virus is found from southern North America to South America, including the Caribbean, and has been observed in several Central American countries to reduce yields by as much as 43%.

Leafhoppers can vector more than one of these pathogens at a time, and mixed infections are common.

Symptoms develop about 2 weeks after plants have been infected. They begin as small, isolated chlorotic spots easily observed by holding leaves against the light (Photo 101). Later, the spots become more numerous and fuse, forming 5 to 10 cm stripes that advance along the veins (Photo 102). If infected at tasseling, plants may not show symptoms. Poor grain set and grain filling are observed with infection at the seedling stage.

107 Maize bushy stunt (MBS) Maize Bushy Stunt phytoplasma, syn. Maize Bushy Stunt mycoplasma This disease has been reported in several countries from the southern USA to Argentina. The pathogen is transmitted by the cicadellid leafhoppers Dalbulus maidis, D. elimatus, and other species of Dalbulus and will be transmitted for most of the life of the vector, after it feeds on an infected plant. The same vectors can transmit MRFV and the corn stunt spiroplasma, where mixed infections in the plant are common.

The pathogen is a non-helical mollicute known as a phytoplasma, earlier referred to as a mycoplasma. MBS is more common in relatively cooler areas, whereas corn stunt is favored by hot and humid environments.

Infected plants show diverse symptoms, depending on the maize genotype. The most common symptoms are marginal chlorosis on young leaves, and tips gradually turn purple-red as they approach maturity.

A conspicuous symptom is the bushy appearance due to a proliferation of tillers, which also become chlorotic and reddish (Photo 103). It is more common in highland germplasm. Foliar symptoms are more obvious close to flowering time. Axillary buds develop into barren shoots.

With early infection ears are produced at many nodes, but with reduced ear diameter and grain size, greatly reducing yield (Photo 104).

Simple observation in the field will not allow one to distinguish between symptoms caused by the maize bushy stunt phytoplasma and those resulting from corn stunt spiroplasma.

108 103 104 109 105 106 110 Corn stunt (CS) Spiroplasma kunkelii, syn. Corn Stunt Spiroplasma The disease is known in hot humid lowlands of several countries of Central and South America, the Caribbean, southern USA, and Mexico, but can also be found up to elevations of more than 2000 meters above sea level. The disease is transmitted by the cicadellid leafhoppers Dalbulus maidis, D. elimatus, and other, less important Dalbulus spp. Vectors can transmit the pathogen for most of their lives after feeding on an infected plant. The same vector can tranmit MRFV and MBS, and mixed infections are common. The pathogen is the helical mollicute Spiroplasma kunkelii.

Infected plants show diverse symptoms, depending on maize genotype, the most common being leaf reddening or purpling, yellowing (Photo 105), and the presence of chlorotic stripes at the base of younger leaves, which might turn purple-red toward the tip (Photo 106). Foliar symptoms normally appear close to flowering time. Plants are stunted due to the shortening of internodes; axillary buds develop as barren shoots or ears at many nodes, and there is excessive root branching. In severe cases, plants are barren, or there is a significant reduction in ear diameter or poor seed set. Plants die prematurely.

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Stalk Black discoloration of stem; Charcoal rot 57 shredding of interior;

bundles of black material.

Narrow elongated brown Anthracnose stalk rot 54 lesions on the stem turning black, wilting with dark brown shredded vascular tissue.

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Brown vascular bundles Black bundle disease 53 extending across the and late wilt internodes starting in roots; wilting of plant beginning at flowering.

Chlorotic and reddish leaves; Maize bushy stunt 108 stunting; plant may be bushy with many tillers.

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Stunting, shortened Corn stunt 111 internodes; axillary bud development; excessive root branching; leaf reddening and/ or marginal yellowing, chlorotic streaks at bases of leaves.

Twisting; dark internodes, soft Pythium stalk rot 45 and water soaked; lodging.

Water-soaked, dark areas Bacterial stalk rot 84 at base of stalk; unpleasant odor, lodging.

Leaf Downy growth on upper or Downy mildew 5 lower leaf surface, striping, partial leaf symptom or general chlorosis; narrow, abnormally erect leaves.

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Lesions, beginning as Gray leaf spot 34 small, regular, elongated brownish gray necrotic spots, and growing parallel to the veins.

Lesions, coalescing to Septoria leaf blotch 38 produce severe blotching and necrosis.

Lesions, elongated, Turcicum leaf blight 17 spindle-shaped, and necrotic; may coalesce to “burn” plant.

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Lesions, oval, necrotic, Yellow leaf blight 25 and parallel to the veins, later blighting the leaf.

Lesions, oval, zonate, Carbonum leaf spot 21 and brownish, or brown slender and elongated.

Lesions, pale green along veins Bacterial leaf stripe 88 developing to stripes, later becoming grayish white to brown and shredding.

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Lesions, small, light brown in Maydis leaf blight 18 color, elongating along secondary veins, and often coalescing.

Lesions, water-soaked with Stewart’s wilt 87 irregular margin spreading along veins, often turning yellow and moving to the stem.

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Spots, small, oval, and water- Turcicum leaf blight 17 soaked, enlarging to elliptical necrotic lesions.

Spots, small and pale green, Phaeosphaeria leaf spot 29 round to slightly elongate later becoming bleached, then necrotic, with a dark brown margin.

Small, round spots on leaves Hyalothyridium leaf spot 30 turning to brown necrotic lesions (3-4 cm) with concentric rings surrounded by chlorosis.

Necrotic, elongated bicolor Borde blanco 13 lesions on the border of the leaf with small white mushrooms formed on the lower surface.

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Striping, chlorotic; Maize mosaic virus I 99 leaves appear crowded and erect; leaves rough, fleshy, dark purple.

Striping, white to yellow and Corn stunt 111 broad, turning purple toward leaf tips, leaf reddening and/or marginal yellowing.

–  –  –

Damaged kernels with Aspergillus ear rots 64 black, yellow-green, ivy green or whitish, powdery spore masses.

Starts as fine stripes going Maize stripe virus 100 often to broad bands of chlorosis.

Often the top of the plant bends.

Underdevelopment of ear.

–  –  –

White-gray fungal growth Gray ear rot 75 between kernels; husks bleached and adhering to each other, later turning black with sclerotia.

White fungal growth; gray- Stenocarpella ear rot 83 brown ear; black pycnidia ;

husks adhering to ear.

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119

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