GOSS'S BACTERIAL WILT AND LEAF BLIGHT

Symptoms

Early leaf symptoms are oblong or elongated lesions of water soaked, greyish-green tissue that progress to long dead streaks. Dark green to black water-soaked spots, called freckles, often develop near lesion edges. A shiny exudate is visible on the lesions where bacteria  oozes out on the leaf surface and may appear shiny after drying to form a glistening residue, or varnish. Plants may also be infected systemically by Goss’s wilt, especially in the seedling stage. These plants have discolored vascular tissue with a slimy bacterial exudate in the stalk.

Plants are commonly stunted and wilt and die as if drought stressed.

Favourable Weather

Bacterial invasion requires an injury to the plant often caused by wounding from wind, sandblasting and especially hail which provide openings for bacteria. Infection is spread through the canopy by rain splash, irrigation and wind.  Goss’s wilt overwinters in residue of maize and several grass species. 

Causal Organism

Bacterium: Clavibacter nebraskensis (Clavibacter michiganensis subsp. nebraskensis)

BOIL SMUT

Symptoms

Galls replace kernels of ear; galls semi-fleshy, initially firm and silver-grey then turn black; membrane ruptures releasing brown powdery spores.

Favourable Weather

The fungus can survive for long periods of time in the soil or crop residues. Spores are carried by air currents or splashed by water to young, actively growing plant parts. Infection normally takes place through the silks or wounds created by hail, insect feeding or other mechanical injury to any part of the plant.

Causal Organism

Fungus: Ustilago zeae (Ustilago maydis)

CRAZY TOP

Symptoms

The most characteristic symptom is the partial or complete replacement of normal floral parts by small leaves.

Favourable Weather

Oospores present in soil are the main inoculum source of S. macrospora. Infection occurs when maize plants are subjected to saturated soil conditions for 24 to 48 hours from planting to about the 5-leaf stage of growth. Pooling of soil and water in the whorl of small plants can also lead to infection. Infection occurs at a wide range of soil temperatures. 

Causal Organism

Fungus: Sclerophthora macrospora

DIPLODIA EAR ROT

Symptoms

Dense whitish fungal growth will be matted between the kernels and between the ear and the husks; begin at base of ear progressing toward the tip; small, black fungal fruiting bodies embedded in the cob tissue and kernel bases.

Favourable Weather

Dry weather early in the season followed by abnormally wet conditions just before and after silking favours ear infection, while a wet early season followed by drier conditions or heat stress later is likely to result in more Diplodia stem rot. Ears are most susceptible from silking to about three weeks later. Hybrids with poor husk cover or thin pericarps are often very susceptible. 

Causal Organism

Fungus: Stenocarpella maydis ( = Diplodia maydis)

FUSARIUM EAR ROT

Symptoms

Dense whitish fungal growth will be matted between the kernels and between the ear and the husks; begin at base of ear progressing toward the tip; small, black fungal fruiting bodies embedded in the cob tissue and kernel bases.

Causal Organism

Fungus: Fusarium verticilliioides = F. moniliforme (Gibberella fujikuroi)

GIBBERELLA EARROT

Symptoms

Pink to reddish mould usually starting at the tip of the ear; mould growth developing between husks and ears; superficial black specks develop on the husks and ear shanks.

Favourable Weather

Maize ears are generally susceptible only when they are very young. Cool, wet weather within three weeks of silking favours disease development. The fungus infects silks, grows into the ear progressing downward during grain fill.

Causal Organism

Fungus: Gibberella zeae (Fusarium boothii)

HEAD SMUT

Symptoms

Floral structures partially or completely replaced by sac-like structures containing masses of black spores; tassel proliferates to form bizarre leafy structures; ears are smutted or aborted, small, round to teardropshaped, with no evidence of a cob or kernels inside and lack silks.

Causal Organism

Fungus: Sphacelotheca reiliana

BACTERIAL LEAF STREAK

Symptoms

Lesions begin at edges or midrib of leaves; lesions enlarge becoming elongate, irregular spots or streaks which are brown surrounded by a yellow border; lesions run parallel to and are confined by the leaf veins.

Favourable Weather

Within fields, it is likely that X. campestris pv. zeae is most effectively disseminated by wind-driven rain or irrigation, but the pathogen may also be disseminated by aphids, wind and plant-to-plant contact.

Causal Organism

Bacteria: Xanthomonas campestris pv. zeae

COMMON RUST

Symptoms

Pustules form on both upper and lower sides of leaf; brownish-red oblong pustules; rust band formed close to the base of leaf.

Favourable Weather

Moderate temperatures (16-25°C) and high relative humidity (>95%), associated with mist or dew for prolonged periods, favour rust development and spread.

Causal Organism

Fungus: Puccinia sorghi

DIPLODIA LEAF STREAK

Symptoms

Brown lesions with characteristic yellow halo; develop from an oval centre extending along the veins; black fruiting bodies develop within the centre of the lesion.

Favourable Weather

More prevalent in warm, humid regions. Unbalanced fertility, low K, poor drainage, mechanical and insect damage, cultivar and planting density all influence disease severity.

Causal Organism

Fungus: Stenocarpella macrospora = Diplodia macrospora

EYESPOT

Symptoms

Small, circular to oval lesions with tan-cream centres surrounded by a brown-purple margin and distinct yellow halo. Easily confused with physiological changes and genetic leaf spots.

Favourable Weather

Favoured by long periods of cool (10-12°C), moist weather during the growing season. Conidia are dispersed by wind or light rain to the leaves of nearby young maize plants.

Causal Organism

Fungus: Aureobasidium zeae = Kabatiella zeae

GREY LEAF SPOT (GLS)

Symptoms

Lesions tan becoming grey as they mature; lesions typically rectangular, limited by and running parallel to the veins; starts on lower leaves progressing upwards.

Favourable Weather

Moderate to high temperatures and high humidity (>95%) favour disease development. The increased adoption of reduced tillage has increased the frequency and severity.

Causal Organism

Fungus: Cercospora zeina

MAIZE STREAK DISEASE (MSV)

Symptoms

Long streaks parallel to the leaf veins developing to alternating dark and light green variegated stripes; stunting; infected plants yellow-green colour; deformed cobs.

Favourable Weather

MSV is only vector transmitted, primarily by Cicadulina spp. leafhoppers of which C. mbila, is the most prevalent. MSV is not transmitted mechanically, by pollen or via seed.

Causal Organism

Virus: Maize streak virus

NORTHERN CORN LEAF BLIGHT (NCLB)

Symptoms

Lesions elongate becoming elliptical or cigar-shaped; grey-green becoming tan in colour with distinct dark zones of fungal sporulation; lesions often surrounded by pale green, water-soaked border.

Favourable Weather

NCLB is favoured by moderate temperatures between 18-27°C and prolonged periods of leaf wetness. Conidia are produced on old residue and are carried by the wind or rain to lower leaves of young maize plants.

Causal Organism

Fungus: Exserohilum turcicum (Setosphaeria turcica)

PHAEOSPHAERIA LEAF SPOT

Symptoms

Bleached, dried, circular to elongate spots with dark brown margins, similar to paraquat herbicide (trade name – Gramoxone) damage; pinpoint, black fruiting bodies develop within lesions.

Favourable Weather

Lesions are usually first visible on the edges of maize fields or on the top leaves of plants. Usually occurs towards the end of the season as winter approaches and cold conditions prevail.

Causal Organism

Fungus: Phaeosphaeria maydis and Phoma sorghina

POLYSORA RUST

Symptoms

Light orange to cinnamon-red, circular to oval raised pustules which are densely scattered over the upper leaf surfaces.

Favourable Weather

Favoured by warm and humid (extended dew or high humidity) conditions, with an optimal temperature range between 24-28°C. In temperate regions, Polysora rust occurs towards the end of the season.

Causal Organism

Fungus: Puccinia polysora

SORGHUM DOWNY MILDEW

Symptoms

Infected sometimes stunted; white-striped leaves; leaf chlorosis includes base leaf; transverse margin sharply defining diseased and healthy tissue (‘half-leaf symptom’); leaves narrower and more erect.

Favourable Weather

Oospores survive in soil and attack susceptible seedlings through underground parts, resulting in systemic infected. Requires a saturated atmosphere or free water and moderate temperatures of 21-25°C.

Causal Organism

Fungus: Peronosclerospora sorghi

CHARCOAL ROT

Symptoms

Black discolouration of lower stem internodes; shredding of interior; numerous minute black specks (microsclerotia) visible on vascular strands.

Favourable Weather

High soil temperatures (30-42°C) and low moisture or when unfavourable environmental conditions stress the plant, especially during postflowering period.

Causal Organism

Fungus: Macrophomina phaseolina

DIPLODIA STALK ROT

Symptoms

Brownish pith discolouration at basal internodes; black specks embedded in rind tissue near rotted internodes; pith disintegrates, weakening stalk predisposing plants to lodging.

Favourable Weather

Infection usually takes place 2-3 weeks after silking under favourable conditions. Wet, early season followed by drier conditions or heat stress is likely to result in more widespread Diplodia Stem Rot.

Causal Organism

Fungus: Stenocarpella maydis (= Diplodia maydis)

ERWINIA STALK ROT

Symptoms

Sudden wilt and lodging; water-soaked, dark areas at base of stalk; most distinctive symptom of Erwinia infection is the foul smell of diseased, slimy plant parts.

Favourable Weather

Favoured by temperatures of 32-35°C and high relative humidity – high rainfall and/or irrigation with surface water through overhead sprinklers; severe when water collects in whorl and on lands subjected to flooding.

Causal Organism

Bacteria: Erwinia pectobacterium pv. zeae. (before Erwinia chrysanthemi pv. zeae.)

FUSARIUM STALK ROT

Symptoms

Premature senescence and lodging; whitish-pink pith discolouration often confused with the red colour of Gibberella stalk rot; shredding of internal stalk; black fruiting bodies absent.

Favourable Weather

F. verticilioides is most common in warm, dry areas and typically occurs in complexes with other root and stalk rots. Infection of plants facilitated by wounds caused by hail or insects.

Causal Organism

Fungus: Fusarium verticilioides = F. moniliforme (Gibberella fujikuroi)

GIBBERELLA STALK ROT

Symptoms

Root and stem lodging; lower internodes turn straw-coloured, internal reddish discolouration, superficial black fruiting bodies on stalk, often at the internodes.

Favourable Weather

Favoured by cooler, moist weather. Stalk infection usually occurs shortly after pollination, developing at the origin of the leaf sheaths or around the prop roots.

Causal Organism

Fungus: Gibberella zeae (Fusarium graminearum)

PINKING

Symptoms

The most characteristic symptom is a general light-pink superficial discolouration of the cob as a result of the production of a red pigment, anthocynin. The discolouration is more noticeable when the cobs are exposed to sunlight. The discolouration is limited to the pericarp, and does not affect meal colour after milling. The pink discolouration may be confused with Fusarium spp.

Favourable Weather

Certain white hybrids are more prone to pinking under specific climatic conditions. Grain may be downgraded at silos and milling plants, having direct financial implications.

Causal Organism

Pinking is a physiological disorder resulting from environmental conditions, occurring sporadically. Limited information is available on the specific temperature and humidity that predispose cobs to pinking.

POPPING

Symptoms

Popped kernel is an irregular break in the seed coat over the kernel crown. The kernel resembles a partially expanded popcorn kernel. The crack in the seed exposes starch providing the ideal substrate for secondary, saprophytic fungi e.g. Fusarium, Aspergillus and Penicillium spp.

Favourable Weather

Popping seems more prevalent in genotypes with hard, flinty kernels. It is common in breeders’ nurseries, but rarely seen on commercial hybrids.

Causal Organism

Popping is believed to be the result of irregular growth, especially since it is common in years with irregular rainfall and particularly when conditions are very hot and dry.

SILK-CUT

Symptoms

Silk-cut is characterised by a horizontal crack over the face of the kernel. The name silk-cut is probably derived from silks, which often become embedded in ruptured areas on the tip of the kernel or occasionally on sides of kernels between the kernel rows.

Favourable Weather

As in the case of popping, the exposed starch also gets colonised by various fungi. This secondary colonisation enhances the kernel damage. This phenomenon is rarely seen on commercial hybrids.

Causal Organism

A dry period followed by wet weather, before full maturity, results in resurgence of deposition of dry matter in the kernel – this stresses the seed coat which cracks and breaks. Another theory is that viable silks of unpollinated ovules push against the developing pericarp of kernels and may also cause this rupturing.

SILK JAMMING

Symptoms

Stresses slow silk elongation causing a delay or failure of silks to emerge from the ear shoot. If the delay is long enough, pollen shed may be almost or completely finished before receptive silks are available resulting in nearly blank or totally blank ears.
 

Favourable Weather

Result from heat and/or moisture stress or other stresses before or during pollination. Excessive plant populations and nutrient deficiency may also encourage this disorder. This phenomenon is also common in hybrids with tight husks when cold night temperatures are followed by warm nights during silking.

Causal Organism

GENETIC STRIPE, LEAF SPOTS AND FLECKS

Symptoms

Some genes alter or limit the ability of the maize plant to produce chlorophyll and result in albino plants or plants with white to yellow stripes or other variegations. Some leaf spots and mosaic patterns mimic symptoms caused by infectious diseases.

Causal Organism

Genetic disorder – genes cause abnormal morphology and colour pattern in plants.

PRE-GERMINATION

Symptoms

Pre-germination of kernels occurs during the latter part of grain filling and is normally only noticed at harvesting.

Favourable Weather

Pre-germination has been observed in certain seasons in KwaZulu-Natal and Mpumalanga.

Causal Organism

Predisposition to premature germination has been shown to be genetically controlled and is also affected by various environmental factors. This phenomenon normally occurs during seasons with low sunlight hours, lots of overcast weather and low heat units. In addition, high temperatures during the later stages of grain growth may break embryo dormancy allowing kernels to germinate on the ear if rain occurs around harvest time. Some ear diseases, such as Diplodia Ear Rot, may induce pre-germination of kernels. Furthermore, molybdenum deficiencies in the grain may also be responsible, the severity of which is enhanced by heavy, late dressings of nitrogenous fertiliser.