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Nature and Source of Inoculum of Aspergillus niger Causing the Aspergillus Black Mold Disease of Onions in New York 2000

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Project Leader: James W. Lorbeer

Objective: The research was conducted to determine the nature and source of inoculum of A. niger as related to the methods of infection by the pathogen on onion seed stalks and flower parts leading to surface infestation/internal infection of onion seed and the subsequent infection of onion seedlings and ultimately of mature plants. Another aspect of the research as initially planned was to identify possible infection pathways of A. niger on onion plants and on onion bulbs late in the growing season immediately prior to harvest, during harvest, and in storage. Because of reduced funding the research conducted was limited to: (A) onion flower and seed infection as well as infection of onion seedlings and young plants by seedborne and soilborne inoculum; (B) identification and use of a selective medium for soil population studies of A. niger in organic soils cropped to onion; and (C) a cooperative study with IPM personnel relating soil population levels of A. niger to lifting, undercutting, and windrowing field grown onions to subsequent levels of black mold.

Discussion: Onion seed became infested with A. niger after the fungus had first infected the flower parts during the period after the sheath had unfolded from the umbel and the florets were still closed until capsule formation. Since the seeds became infested on the surface and not infected internally, it is possible that when this situation occurs, a seed treatment with a specific fungicide prior to planting could be helpful in reducing the amount of onion seedling infection by A. niger. Also, measures that prevent onion flowers from becoming exposed to A. niger during the stages when they are most susceptible to infection should reduce the incidence of A. niger on onion seed. Whether seed infestation or infection by A. niger and/or direct infection of onion seedlings by soilborne A. niger and then subsequent systemic infections of young plants can lead to black mold in mature onion bulbs remains to be determined. Since A. niger repeatedly has been detected in most seedling parts grown from seeds infested with the fungus prior to sowing, this indicates that the fungus is capable of becoming endophytic (alive but not producing disease symptoms) in seedlings and young onion plants. Whether the fungus infects the seedling as the seed germinates or rather through roots or other plant parts after germination also remains to be determined.

In the six fields cropped to onion in Orange County that were tested for levels of A. niger at three different times in the growing season using PLYA, the population levels of the fungus increased significantly from May to July when the onions were bulbing. The rise in population levels of A. niger in those soils may have coincided with growth stages of the onions and could have been affected by weather conditions and/or management practices. Since, no correlation was found between soil population levels of A. niger and fields with a history of black mold, this suggests that even in the presence of the pathogen, environmental conditions ultimately regulate the occurrence of black mold.

In the cooperative trial with the IPM personnel involving onion field drying practices (lifting, undercutting, and windrowing), black mold levels at Orange County onion farms generally were very low during 2000 due to the rather mild cool weather that occurred during the growing season and at harvest time. However, the field in which one of the onions with black mold was grown was the only one of the seven in the trial in which black mold was observed both on onion plants throughout the field (inside and outside the trial areas) prior to harvest and in onions after storage. This field also had the highest level of A. niger in the soil (3.9 x 103 CFU or propagules) of the seven fields sampled. When seeds from seed lots of the seven varieties from the 1999 variety trials which had differing levels of black mold in storage were assayed, only a low level of A. niger was detected. This suggests, while seeds may be an important source of inoculum, airborne and particularly soilborne sources may be important as well. Additional factors such as weather conditions during the growing season and at harvest along with storage conditions also should be taken into account when determining factors regulating the occurrence of black mold.

The findings in 2000 provide a clearer understanding of the biology of A. niger, especially as a soilborne pathogen. However, they also raise important questions. Although onion seeds, seedlings, and young plants can become systemically infected with A. niger, how important is this to the subsequent occurrence of black mold on mature onion plants just prior to harvest and onion bulbs in storage? What is the effect of different soil population levels of A. niger under favorable environmental conditions for the pathogen on black mold occurrence at harvest time and in storage? It is beginning to become apparent that even with the presence of A. niger as an endophyte in onion plants and as a soilborne pathogen, the environmental conditions of high temperature and moisture ultimately regulate the pathogenesis of the fungus on onion plants and bulbs.