Differential lines and controls: seed handling and seedling raising

A separate and rust-free area must be maintained to keep seed materials used for race identification clean and free of contaminants. Seeds of each differential and susceptible check must be stored in clearly labeled, separate glass or plastic containers with lids, and kept in a cool area — cold room or refrigerator, if available (Fig. 2).

For raising seedlings, use sterilized soil composed of three different materials: soil, sand, and farm yard manure mixed at the ratio of 2:1:1 by volume. Seedlings are raised by two methods.

• One method is to pre-germinate seeds by spreading the seeds on filter paper in petri dishes, moisten with water and close the lid. On the third day, the seeds will germinate and the radicles are seen. Then these germinating seeds are planted in pots using forceps (Fig. 3).
• A second method is direct planting of the seeds in pots filled with sterilized soil. More uniform seedling germination is normally achieved by using the first method (Fig. 4). In both methods, at 7-8 days after planting, the primary/first leaf will be fully extended and the second leaf will be 2-4 cm long. This is the right stage for inoculation with rust pathogen spores in the greenhouse.

Seeds of a highly susceptible host (for example, McNair) are planted to multiply the rust collected from the field sample to be analyzed. Once the sample is increased sufficiently, the 20 current standard stem-rust differential lines (Table 1) are planted for race designation. It is critical that the seedling raising-room be free from rust spores. Rust-free protective gowns must be worn when activities are going on in the room. Different sized clay and plastic pots are used according to the needs of the activity to be conducted. Trays are used to group the pots in order (Fig. 5).

Table 1: Wheat Stem Rust Differential Lines (accessions deposited at the National Plant Germplasm-Aberdeen, ID)