Crown infection of corn by Diplodia zeae

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McNew, George
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The symptoms of crown infection of corn caused by Diplodia zeae include: a dark straw-brown discoloration of the tissues of the crown and lower internodes; prevalence of subepidermal pycnidia on the crown and around the aerial adventitious roots; disintegration and shredding of the internal tissue of the crown; intense brown discoloration of the nodal plates, and a dark brown decay of the mesocotyl which results in the loss of the primary root system.

Crown infection occurred on 14 to 52 percent of the field corn in central Iowa in the years from 1930 to 1933.

Practically every corn plant grown from Diplodia-infected seed may show crown infection at maturity. Infected seedlings which, by means of timely establishment of adventitious roots, do not die may be parasitized during the remainder of the growing season.

The symptoms on plants grown from disease-free seed in Diplodia-infested soil are identical with those derived from infected seed. The pathogene grows through the soil to the mesocotyl where it becomes established in the wounds created by the emerging seminal roots. The crown may be invaded directly from the soil at the point of emergence of an adventitious root but the crown is usually invaded from the decayed mesocotyl.

The pathogene progresses up the mesocotyl from this primary lesion and becomes established on the crown. The mycelium spreads internally to a limited extent during the growing season and rapidly involves the entire crown and lower nodes at maturity.

Although Diplodia zeae spreads from the mesocotyl upward for several nodes, it is not systemic in the plant. Under field conditions it is generally restricted to the first internode above the roots.

Crown infection has been induced by infesting steamed soil with infected crowns and stalks of plants which had over-wintered under field conditions.

Diplodia zeae may live in a soil devoid of plant refuse but its development is hampered in mixed culture. The presence of the pathogene in ordinary soil from a corn field has been demonstrated by growing plants from disease-free seed in such soil collected in the spring.

Invasion of the crown from infested soil is most severe at high soil moisture contents, but the development of the infected plants is severely reduced at either high or low moisture contents. The dry weight of plants growing at optimum soil moisture content was not reduced by crown infection.

The dry weight of roots is reduced more than that of the tops of plants grown in infested soil. The dry weight of the tops, however, is significantly less than that of non-infected plants grown under soil moisture conditions approaching the maximum or minimum for growth.

A light infection of a plant growing at low soil moisture may be almost as injurious to the plant as a severe infection where there is an abundant supply of water.

The transpiration ratio of infected plants is increased at soil moisture contents favorable to reduction in dry weight.

Plants grown from Diplodia-infected seed treated with a mercury dust had a higher percentage of crown infection than similar plants from untreated seed. Treatment of disease-free seed with the same dust decreased the amount of crown infection of plants growing under field conditions. The dust apparently inhibited the fungus without killing it in the seed. Plants protected in this fashion later suffer crown infection.

The extent of crown invasion under given conditions depends upon the time of initial infection. Seed treatment of disease-free seed may delay mesocotyl invasion from infested soil and thereby decrease crown infection.

Selfed lines of corn have different reactions to crown infection by Diplodia zeae under field conditions and furnish some remarkably resistant lines for genetical studies.

Late crown infection is a significant phase of the parasitism of Diplodia zeae on corn. The pathogene may reduce the dry weight of the plant to half that of a normal plant under certain soil moisture conditions. Under the most favorable field conditions there is a reduction in yield.