Stenocarpella maydis (ear rot of maize)

RootsInfected plants exhibit necrotic lesions; dwarfing and stunting follow. S. maydis occurs together with other organisms in the root-rot disease complex. Chambers (1982) suggested isolations of S. maydis made from roots could have arisen from secondary invasions, implying that S. maydis is not a primary pathogen of maize roots. Crown infections occur from soil- or seedborne inoculum. Symptoms of crown infection include straw-brown discoloration of the crown and lower internode tissues and prevalence of subepidermal pycnidia on the crown and aerial adventitious roots (McNew, 1937).StemsS. maydis stalk rot manifests itself after flowering, resulting in wilted plants and leaves and eventually death, resembling frost injury. The lower stalk internodes become brown to straw-coloured, soft and spongy and are easily crushed. Stalk pith tissue disintegrates and discolours until only vascular bundles remain intact. Typical symptoms of an infected stalk are subepidermal black-brown pycnidia which cluster at stalk nodes. White fungal mycelium may be observed on the rind surface (Shurtleff, 1980).EarsEar-rot symptoms generally result from shank infections which, under suitable conditions, ramify up into the ear, resulting in generalized ear rot. Silk infections occur with downward mycelial ramification, but such infections are less frequent (Koehler, 1942). Husks of early infected ears die and become straw-coloured while the plant remains green. Infections which occur within 2 weeks of silking cause rotting of the entire ear which becomes shrunken, light in weight and turns greyish-brown. The mycelium ramifies throughout the ear and husks causing them to adhere tightly to one another. This lightweight ear remains upright until harvest (Shurtleff, 1980). The leaf sheath and leaf directly below an early infected ear may die (Flett et al., 1996). Pycnidia develop readily on husks, cob tissue and maize kernel pericarp.Later ear infections show no external symptoms, but removal of kernels exposes a white mould growing between kernels which develop discoloured embryos (Shurtleff, 1980). Occasionally early infections may occur, but unfavourable climatic conditions may inhibit mycelial ramification. Similarly, late infections may reduce the rate of mycelium ramification. These result in the only symptoms being a slight discoloration of kernel embryos, termed 'hidden diplodia' in South Africa (Koehler, 1942; Flett et al., 1996). Some isolates of S. maydis induce vivipary (premature germination) of kernels on the maize ear (Calvert et al., 1969).

Host-Plant Resistance

Stalk-rot-resistant genotypes have been reported extensively in the literature (Hooker, 1977; Fakorede and Mock, 1978; Shurtleff, 1980; Clark and Foley, 1985; Chambers, 1987; Wicks et al., 1988; Coors and Mardones, 1989). These reports concentrate largely on lines and experimental material, with very few reports of hybrid and variety differences.

Loesch et al. (1962) found S. maydis stalk rot did not affect rind thickness but did affect stalk-crushing strength. Anderson and White (1994) found rind puncture at anthesis ranked most consistently with lodging and premature death. Rind thickness, pith density, pith moisture and the stalk push test were inferior indicators of resistance compared to rind puncture. Choi et al. (1994) found stay-green lines were resistant to S. maydis stalk rot. Unfortunately, breeders have concentrated on breeding for rind thickness to improve standability and not for S. maydis resistance. Breeding for S. maydis resistance would reduce stubble-borne inoculum sources.

Inheritance of Diplodia stalk rot resistance is quantitative with a predominantly additive gene action (Bansal, 1969). Recurrent selection has been the most commonly used breeding approach for S. maydis stalk rot improvement (Martin and Russell, 1984; Lu and Lambert, 1988; Nyhus et al., 1988; Grombacher et al., 1989; Nyhus et al., 1989).

Young (1943) developed a toothpick method for inoculating maize ears and stalk rots. Although this method was used extensively, morphological resistance mechanisms were bypassed. Warren and Von Qualen (1984) developed a more natural inoculation technique for evaluation of stalk-rot resistance. Based on the theory of Dodd (1980), Melis and Rijkenberg (1988) developed a moisture-stress technique for screening maize hybrids for stalk-rot resistance by placing a heavy-gauge polythene sheet under the soil with a drain. The ensuing poor root development plus water drainage created moisture stress and maize stalk rot increased.

Ear-rot resistance in breeding material (Koehler, 1953; Wiser et al., 1960; van Rensburg and Ferreira, 1997) and hybrids (Rheeder et al., 1990b, c; Flett and McLaren, 1994) have been reported. Resistance to Diplodia ear rot showed significant general and specific combining ability effects with no reciprocal effects (Dorrance et al., 1998). Inbred lines B37, H111, B68 and MS were identified as good combiners increasing progeny resistance. Flett and McLaren (1994) explained inconsistencies observed in hybrid resistance rankings by regressing the hybrid actual ratings to the trial means (potential). Hybrids reacted non-linearly to different disease potentials. Above 50.6% disease potential, the use of S. maydis ear-rot-resistant hybrids was insignificant in controlling disease. Opaque-2 genotypes are more susceptible to maize ear rot (Ullstrup, 1971; Loesch et al., 1976), but Gevers et al. (1990) found high levels of polygenic ear rot resistance in F and M heterotic groups in an opaque-2 nursery.

Wiser et al. (1960) found resistance to S. maydis ear rot was manifested by the ability to resist initial infections rather than at later developmental stages. Further resistance mechanisms documented include physiological resistance, husk protection and ear declination (Koehler, 1953).

Ullstrup (1949) criticized the toothpick technique developed by Young (1943), on the basis that wounding would nullify the effects of structural or physiological barriers; he consequently developed a technique for spraying a spore suspension on ears from silk to shank. Ullstrup (1970) found the severity of Diplodia ear rot was proportional to spore concentration. Klapproth and Hawk (1991) compared various techniques and selected as the best spraying of silks with a spore suspension. The highest production of pycnidiospores and mycelium was on oat flour agar after 25 days incubation (Bizzeto et al., 2000). Warren and Von Qualen (1984) inoculated maize whorls with a spore suspension, but this technique resulted in high variation under South African conditions, possibly due to dessication of the spore suspension before infection (BC Flett, ARC-Grain Crops Institute, South Africa, personal communication, 1996). Flett and McLaren (1994) used a technique where infected kernels were ground and applied to the whorl 2 weeks before tasselling, with satisfactory results. Bensch (1995) compared various techniques and found that both spore suspension applied in the leaf sheath and ground infected kernels in the whorl were successful methods. Bensch and Flett (1996) showed that both techniques resulted in adequate ear rot, but the ground, infected kernel technique had a longer infectious period (4-12 weeks) than the spore suspension (9-12 weeks).

Cultural Control and Sanitary Methods

Crop rotation
Flett (1991) confirmed that maize is the only crop host of S. maydis. Reduction in pycnidia and reduced spore viability of surface stubble after 24 months implied that crop rotation should be effective in Diplodia stalk- and ear-rot control (Flett, 1995a). Crop rotation did not reduce stalk rot of maize (Wilcoxsen and Covey, 1963), but these plants were inoculated and natural infection effects were overridden. Flett (1995b) found Diplodia ear rots to be reduced by a single-season rotation away from maize when compared to monoculture maize. Wheat, soyabean and groundnut were the most effective rotation crops with sunflower the least effective for reducing S. maydis ear rots (Flett et al., 2001).

Tillage practices
Hopkins (1939) suggested that to control Diplodia stalk and ear rots the bottom three stalk nodes should be destroyed by ploughing early in the season, and stubble burial should be ensured. Byrnes and Carrol (1986) reported increased Diplodia stalk rot on no-tillage maize planted on heavy soils. Diplodia ear rots were significantly reduced by ploughing-in infected stubble when compared to minimum and no-tillage systems. A linear relationship between surface stubble mass and S. maydis-infected ears was obtained (Flett and Wehner, 1991). Flett et al. (1998) studied the efficacy of periodic ploughing in reduced tillage fields to reduce S. maydis ear rots. Mouldboard ploughing significantly reduced S. maydis ear rot but on reverting to reduced tillage S. maydis ear rots increased significantly in the following season. Thus, alternating tillage practices will not reduce S. maydis ear rots in the long term.

Soil fertility
Soil fertility imbalances are conducive to Diplodia stalk rot (Koehler, 1960). Stalk pith conditions were poor in plants treated with nitrogen or phosphorus. Addition of potassium or limestone decreased pith damage (Pappelis and Boone, 1966). Otto and Everett (1956), Foley and Wernham (1957), and Parker and Burrows (1959) also reported that potassium applications reduced stalk rot. However, Josephson (1962), Krüger et al. (1965) and Abney and Foley (1971) found no response, possibly due to high potassium levels before the studies as stated by these authors. High N:K ratios increased stalk rot and low N:K ratios reduced stalk rot (Foley and Wernham, 1957; Parker and Burrows, 1959). N:K ratios greater than 3.5 showed rapid stalk parenchyma breakdown (Liebhart and Murdock, 1965). Huber and Watson (1974) found that the form of nitrogen available affected stalk-rot severity whereas nitrapyrin, a nitrification inhibitor, decreased stalk rot (Warren et al., 1975). Nelson (1963) found susceptible maize hybrids had greater Diplodia stalk rot when treated with nitrate than with ammonium fertilizers. In conclusion, a balanced fertilization programme using ammonium as a nitrogen source should reduce Diplodia stalk rot.

Stress reduction
From Dodd's (1980) translocation stress theory, any stress reduction will reduce stalk rot. Reduction of plant populations reduces moisture stresses and in turn reduces Diplodia stalk rot (Krüger et al., 1965).

Planting density
Sangoi et al. (2000) screened a number of maize hybrids under different plant densities and found that increasing densities increased the incidence of stem rots particularly on certain hybrids which lodged excessively prior to harvest. This was not the case for Diplodia ear rot, however, where hybrids varied in susceptibility but plant density had no significant effect.

Stalk RotChambers (1987) found no correlation between stalk rot and yield loss in an artificially inoculated study. However, Littlefield and Wilcoxsen (1962) and Wilcoxsen et al. (1963) found a significant reduction in grain yield where necrosis of the second internode involved 50% or more of the tissue. In the USA, Christenson and Wilcoxsen (1966) estimated annual yield losses of 5-20% due to stalk rot and lodging. Differences in grain weight between stalk-rotted and healthy plants in a naturally infected field ranged from zero to 26.2%. Relating these data to disease incidence in Illinois, state-wide losses were estimated to be 8.6%, valued at US$ 70 625 000 annually (Hooker and Britton, 1962). Chambers (1988) explained yield loss inconsistencies due to stalk rot to be related to vascular bundle distribution in different maize genotypes.Ear RotChambers (1988) found yield losses (grain weight per plant) as high as 97% from Diplodia ear rot inoculations made 10 days after silking. Unfortunately, yield losses due to natural ear rot infections have not been quantified. Diplodia ear rot causes grain quality problems which may not result in yield losses but in economic losses.Koehler et al. (1925) reported stand decreases of 36.3% and yield losses of 32.4% due to planting S. maydis-diseased seed. Nwigwe (1974) reported S. maydis to cause between 4.9 and 36.9% loss in germination. Seed germination was negatively correlated with S. maydis-infected kernels (Rheeder et al., 1990a).Mycotoxic EffectsA mycotoxin that affects livestock has been reported from South Africa. Poisoning of cattle fed on Diplodia-infected maize stover was first reported by Mitchell (1919) in South Africa. Of four oxen force-fed infected maize ears, three developed typical diplodiosis symptoms. Isolates able to induce diplodiosis under experimental conditions were obtained from maize grown in the USA, Argentina and South Africa. Isolates toxic to ducklings and rats were not always able to induce diplodiosis in cattle or sheep. This neuromycotoxicosis has been reported only under natural field conditions in South Africa (Rabie et al., 1985). Sheep were reported to have perinatal mortalities when exposed to diplodiosis in the second and third trimesters of pregnancy (Kellerman et al., 1991). Fincham et al. (1991) reported mycotoxic peripheral myelinopathy, myopathy and hepatitis in vervet monkeys caused by S. maydis.