Understanding Ergot

Share this
Advertisement

Understanding Ergot

The term ergot is the common disease name for a group of fungi in the genus Claviceps. Ergot also refers to the typically elongated fungal structure, technically known as a sclerotium. Claviceps is a unique group of fungi, which infect only the ovaries of grasses, replacing what would normally be a seed with a sclerotium. There are some 40 known species of Claviceps. Most are tropical or subtropical and nearly all are restricted to a single genus or closely related genera. Ten species occur in the U.S., but only three are of economic importance. The recently introduced Claviceps africana is a serious pathogen of sorghum in the Midwest and south central states. Claviceps paspali is an important pathogen in seed production of Paspalum species in the southern states. But the species of greatest concern is the widespread and common Claviceps purpurea. This species is unique in having an exceptionally broad host range, including as many as 400 species of grasses, including wheat, rye, barley, and all of the cool season forage grasses.

Biology of Ergot

The sclerotium of Claviceps pupurea is a specialized structure. The outer layer, or rind, provides protection and resists attack from insects and other microbes. The internal tissue is high in fats and nutrients needed to support germination and production of fruiting bodies. The sclerotium overwinters on or in the soil and will not germinate until dormancy within the sclerotium is broken by a period of several weeks of cold temperatures.

Germination begins in the spring. Moist soil is required for germination of sclerotia. Dry conditions will interrupt germination, but it will proceed when wet conditions resume.

The early phase of germination and fruiting of the sclerotium includes production of a stalk. The stalk will be short if the sclerotium is on the soil surface, but if below the surface or under leaf litter, the stalk will continue to elongate, up to 1 to 2 inches, until it reaches light. A specialized spherical structure called a capitulum, no bigger than the head of a pin, develops at the end of the stalk. Embedded within the outer surface of the capitulum are spherical or flask shaped structures called perithecia. Within each perithecium are numerous slender bodies called asci. Each ascus contains 8 ascospores.

Considering the number of asci within each perithecium and the number of perithecia within each capitulum, e capitulum is capable of producing thousands of ascospores. The slender thread-like ascospores are ejected from the perithecia, typically early in the morning, and the microscopic spores, finer than dust particles, are transported with only the slightest movement of air. The production and release of ascospores occurs when there is a high moisture content at the soil surface, typically as the result of rain or irrigation. Given a rain event, ascospore production will often continue for several days, until the soil surface dries. Ascospore production continues when the soil is once again moist.

Advertisement

The size of the sclerotium that develops is determined by the size of the seed of the host plant. Small seeded grasses such as bentgrass or bluegrass produce much smaller sclerotia than large seeded grasses such as wheat and barley. The larger the sclerotium, the more stalked capitula, or fruiting bodies, that will be produced. Kentucky bluegrass sclerotia typically produce 3 to 6 fruiting bodies. Six to twelve fruiting bodies would be expected from a large sclerotium. Sclerotia do not all germinate at the same time. Some will germinate early, some late. This ensures a production over a several month period in the spring, beginning with early flowering grasses. scospore

The timing of ascospore production coincides with flowering in grasses. The feathery stigmas, so effective in trapping airborne pollen, are equally effective in collecting ascospores. The ovary is the only organ of the grass plant susceptible to infection and the fungus will not infect any other part of the plant. In most cases, the fungus enters at the base of the ovary and begins to colonize the tissue. Within a few days of infection a sphacelium, or surface producing spores, develops. As the spores, called conidia, are produced they mix with plant sap leaking from the infection site in what is commonly referred to as the honeydew stage.

As the sap evaporates, sugars and conidia are concentrated into a sticky syrup. The high sugar content prevents conidia from germinating. However, conidia that are transferred to other flowers, especially u flowers, can infect those flowers. Rainy and windy conditions can be effective in spreading the conidia other flowers, contributing to additional infections. About 7 to 10 days after infection, the outer sphacelium is converted into the rind of the sclerotium and the structure begins to take on the characteristics of the sclerotium.

It is important to keep in mind that the longer the duration of flowering the longer the duration that the grass will be susceptible to infection, and the greater the probability of infection. Cool conditions, and especially cool wet conditions that prolong the flowering period, will favor ergot infection and greater disease severity. It is also important to keep in mind that although moisture is required for ascospore production, such conditions are not required for infection from honeydew. In fact, ergot can continue to develop under warm, dry conditions if honeydew is transferred to uninfected flowers. This can occur through head to head contact, movement by insects, or any other means, animal, human, or mechanical that transfers the sticky honeydew from one flower to another. The honeydew is very dense with conidia and even the tiniest amount is enough to infect a flower.

During the past 100 years we have made some progress in our understanding of ergot although disease management continues to be a challenge. There are several approaches to management of ergot, including disease resistance, fungicides, and minimizing primary and secondary sources of inoculum.

Advertisement
Share this

Leave a Reply