Practical Diagnosis of Wheat and Barley Diseases

Collection of Samples

Collections of disease samples are extremely useful in studying disease symptoms and forming representative pathogen populations. The isolates can later be used for artificially inoculating host genotypes in selecting for resistance.
When collecting field samples, it is important to take into account such factors as crop growth stage at the time of collection, plant part to be collected, lesion development, and prevailing environmental conditions. Collection data such as location, date, crop, and variety, as well as symptom description, should be specified for each sample.

Preservation of Samples

Samples can be placed in paper or plastic envelopes, unless the weather is hot and humid, because these conditions favor saprophyte development. Samples should be dried as soon as possible. In general, plastic bags are used only in emergencies or when other types of containers are unavailable.
A press is recommended for drying the samples (see below for instructions on how to build one). Place plant tissue in the press in such a way that symptoms are readily visible. When samples consist of leaves whose adaxial and abaxial sides are different, two leaves are used, one face up and the other face down. Ifa press is not available, samples can be laid out in the same manner on newspaper or blotters and a weight placed on top.
The dried sample and its corresponding data are transferred to a diagnostic sheet. If the necessary equipment is available, samples are observed under the microscope or isolates made, to start the identification process. All observations made during this process (signs and symptoms, morphology) should also be added to the diagnostic sheet (Figure 1; Zillinsky, 1983). In this way, a reference sample of the most important diseases in a region can be made that includes data which later will facilitate pathogen identification and help to understand local phytosanitary problems.
How to build a press. A press consists of two base parts made of a durable, strong
material such as wood (Figure 2a, b). Nail together four wood strips 1.5 inches wide to
form a 12" x 18" (30.5 cm x 46 cm) rectangle, as shown in Figure 2b. (measurements can be adjusted as needed). Place newspaper between the two bases to dry plant samples. To apply and maintain constant pressure on the samples, use two I" (2.54 cm) wide
leather strips for tightening the press (Figure 2b).

Ifsamples are to be used for starting a collection of the pathogen population, dry them for 48 h, making sure they are properly identified. After drying, samples are transferred to paper envelopes, which in tum are placed inside plastic bags. Plastic bags should be sealed to keep out moisture and stored at 4°C.

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