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Biological seed treatment offers a safe, environmentally responsible option for protection of seeds and seedlings from attack by soilborne pathogens. Most effective biological seed treatments have used either bacterial or fungal agents. The efficacy of a biological seed treatment depends upon the ability of the biocontrol agent to compete and function on the seed and in the rhizosphere under diverse conditions of soil pH, nutrient level, moisture, temperature, and disease pressure. Seed treatment performance may be improved through application and formulation technology. An example of this is the bio-priming seed treatment, a combination of seed priming and inoculation with Pseudomonas aureofaciens AB254, which was originally developed for protection of sh-2 sweet corn from Pythium ultimum seed decay. Bio-priming has been evaluated for protection of seed of sweet corn and other crops under a range of soil environmental conditions.
Shrunken-2 supersweet (sh2) sweet corn is susceptible to preemergence damping-off caused by Pythium ultimum, especially when planted into cold soil. Bio-priming, a seed treatment which combines the establishment of a bioprotectant on the seed with preplant seed hydration, was developed to protect seeds from damping-off.
In a series of field experiments conducted in Montana's Bitterroot and Gallatin Valleys, bio-priming or seed bacterization with Pseudomonas fluorescens AB254 protected sweet corn from P. ultimum damping-off. Bio-priming corn seed with P. fluorescens AB254 was comparable to treatment with the fungicide metalaxyl in increasing seedling emergence. Seedlings from bio-primed seeds emerged from the soil more rapidly than from nontreated seeds and were larger at three weeks postplanting. Seeds of sh 2 and sugary enhancer (se) sweet corn, as well as that of several sh 2 cultivars, were protected from damping-off by bio-priming.
In field experiments, bio-priming and coating with Pseudomonas fluorescens AB254 consistently protected sweet corn (Zea mays L.) seeds from preemergence damping-off caused by Pythium ultimum Trow. The bio-priming seed treatment was evaluated under various disease pressures and with seeds of three sweet corn genotypes: shrunken-2 supersweet (sh-2), sugary enhancer (se), and sugary (su). While no damping-off occurred in the su sweet corn, bio-priming protected sh-2 and se sweet corn seeds at a level equivalent to that obtained by treatment with the fungicide metalaxyl. Biopriming increased seedling height of all three sweet corn genotypes at 4 weeks post-planting. Coating of sweet corn seeds with P. fluorescens AB254 provided an equivalent degree of protection from damping-off under all but the most severe conditions.
Penicillium oxalicum is a seed- and soilborne fungal pathogen that causes preemergence damping-off and postemergence seedling blight of sweet corn, While seed infection and infestation by P. oxalicum is common, the amount of injury observed in the field is variable. Our objective was to determine factors influencing the occurrence and severity of disease due to P. oxalicum. Inoculation of sh-2 sweet corn seeds with conidia of P. oxalicum reduced seedling emergence and resulted in seedling mortality. Disease severity in the greenhouse and the field was greater as inoculum density increased from ≈ 102 to 106 conidia per seed. Increasing soil temperatures after planting inoculated seed resulted in more preemergence damping-off. Penicillium oxalicum is capable of growth and sporulation in soil that is too dry for seed germination. Nontreated (naturally infected) sh-2 sweet corn seeds or seeds inoculated with P. oxalicum were incubated in pasteurized soil that had been adjusted to various moisture levels-all too low for seed germination. Increasing soil moisture was associated with visible growth of Penicillium spp. on seed after incubation, and greater levels of damping-off and seedling blight when the seed was planted.
Sweet corn (Zea mays L.) seed is commonly infected or infested with fungi that can impair stand establishment. Among these, Penicillium oxalicum Currie and Thorn is known to cause preemergence damping-off or postemergence seedling blight. Supersweet, or shrunken2 (sh2), sweet corn cultivars are particularly affected by seedborne fungal pathogens, although the effects of seed infection on seedling emergence and stand are variable under field conditions. This study was conducted to examine factors that could influence the impact of P. oxalicum on seedling stand, including P. oxalicum inoculum density on seed and in soil, soil moisture, soil temperature, and control of seed decay caused by soilborne Pythium ultimum Trow. Seed surface disinfestation usually had no effect on seedling stand under conditions favoring infection by P. ultimum. Inoculation of sh2 sweet corn seeds or infestation of soil with conidia of P. oxalicum resulted in increasing severity of damping-off and seedling blight as inoculum density increased. In pasteurized soil in the greenhouse, an inoculum density of 102 P. oxalicum conidia per seed reduced emergence and induced seedling blight. In the field, where P. ultimum was also a factor, 106 conidia per seed were needed to reduce emergence and 105 conidia per seed to reduce healthy seedling stand. When pythium seed decay was controlled by metalaxyl seed treatment, seedling emergence and healthy seedling stand were both reduced at 1 × 106 P. oxalicum conidia per seed. When sh2 sweet corn seed was inoculated with conidia of P. oxalicum and incubated in soil at subgermination moisture contents (4.2 to -7.8 MPa) for 2-4 weeks before planting and irrigating, P. oxalicum reduced seedling emergence at all soil moisture levels, but caused the greatest amount of injury after planting when seeds were incubated in soil above -5.1 MPa. As soil temperature increased from 9-25C, seedling emergence from seed inoculated with P. oxalicum was progressively reduced, with a decrease of nearly 50% at 25 C. Penicillium oxalicum has the greatest potential to reduce seedling stand when infected sweet corn seeds are planted in warm, dry soil, but the effects of this and other seedborne fungal pathogens may be masked under conditions favoring infection by P. ultimum.