Rusts

Family: Pucciniaceae

About Rust Diseases

Life Cycle

Rust diseases are a group of crop diseases caused by fungi. Common types of rust include: stem, crown, stripe, and leaf rust. Generally, all forms of rust occur under similar environmental conditions and display similar symptoms. Derived from the fungal genus Puccinia, rust diseases create small pustules that affect the aerial parts of plants. Leaves and stems are most commonly affected, but rust can also form on some fruit and flowers. Air currents are the primary agent causing the spread of rust diseases. Summer air currents can carry spores known as urediniospores from one field to another in the northern United States and Canada. It is also possible for rust to overwinter as resting spores if spring conditions allow for rust development. After the spores are exposed to moisture on plant leaves for several hours they will germinate and infect the host. Once the spore penetrates the stoma, fungal mycelia spread through the leaves and/or stem causing the distinct yellow-brown appearance of pustules. These symptoms will appear about one week after the initial infection so long as the conditions are suitable. Spores are produced within eight days to two weeks after infection. The rust pustules contain urediniospores which allow for dispersal for further infections. Throughout the growing season, fresh spores can continue to infect green plant tissue. In severe cases, rust may spread to the upper parts of the plant canopy, causing yield reductions. This is because the rust takes up green leaf areas used for photosynthesis and causes water loss through evaporation resulting from the weakened leaf epidermal layer. In the later stages of the season, the rust pustules are replaced by dark brown telispores that do not play a role in infection because telispores are not able to infect any known alternate hosts. The production of urediniospores restarts when mild temperatures and suitable moisture levels appear.

Rust disease Identification and Habitat

Identification

Rust diseases will typically cause spore pustules to appear on green leaves and stems in scattered clusters or stripes. Pustules can appear in yellow, brown, orange, white, or black. Many rusts are a rusty brown color, hence its common name. As rust spreads to the upper canopy of a plant, visible yield reductions may occur. Symptoms are commonly seen in mid to late summer and autumn as air temperatures become mild and moisture levels rise. Plants in areas with cooler temperatures and higher moisture levels are more susceptible in general. Typically rust diseases will not kill the infected plant, but severe infection will dampen the strength of the plant. Heavily-covered leaves often change color and fall prematurely due to water loss. There are extreme cases (e.g. antirrhinum rust) where the plant can be killed. Some species of rust will have a powdery appearance instead of a spotted one.

Varieties

• • Leaf Rust
• • Stem Rust
• • Stripe Rust
• • Crown Rust

Habitat

Rust diseases are able to germinate most effectively in temperatures ranging from 5 to 15 C with periodic rains. Cool temperatures and moisture are necessary for rust spores to germinate and infect. In temperatures ranging from 10 to 15 C, rust diseases will peak in their development. Additionally, conditions with dew and high humidity will favor the development of rust diseases. Furthermore, fields that lack quality sanitation will aid the spread and transfer of spores from plant to plant. Also, the density of crop canopies can impact fungal development, where higher density facilitates development. Overall, rust diseases flourish in environments with cool temperatures, plenty of moisture, and high humidity.

Rust disease Management and Control Methods

Cultural Control

In order to prevent rust diseases, be sure to use only diseased-free seeds for planting. This will help avoid early development of rust diseases in crops. Since rust diseases develop most effectively in moist conditions, controlling weeds is vital for proper air circulation and quick drying of the plants’ canopies. When appropriate, pruning plants properly can help to remove unwanted excess moisture from foliage and it will improve air circulation of the plant. Removing small numbers of infected leaves can help slow down the development of rust. If a farm was heavily infested during the previous crop, crop rotation can help reduce the chances of rust development in further seasons. It is important to provide conditions that encourage growth, but an excessive amount of foliar nitrogen can favor fungal infections. Also, practicing little to no tillage can reduce the chances of rust fungi overwintering in the layer of stubble from former crops. At the end of the growing season be sure to remove all diseased and dead material to avoid overwintering of spores.

Chemical Control

To avoid yield losses, early application of fungicide can be crucial. Be sure to begin inspecting the crop at its seedling stage and continue to inspect regularly. Planning an inspection right before the flag leaf emerges can be very beneficial to prevent further spread of disease. Applying fungicide before rust covers 5% of the leaf surface area on the flag leaf is fundamental in controlling early spread of rust. Before using fungicides, always make sure to read the label carefully for cautions and proper application steps. Although seed treatment can delay rust diseases, airborne spores can still infect the crop later in the season; therefore a foliar application of fungicide is important. When using chemical controls, consider the short-term benefits, but also consider economic and environmental concerns.

Sources