Inspection of Plants beyond the Seedling Stage

Abstract

Certain categories of seed-borne diseases require a longer period of incubation to be detected than is provided by the usual incubation procedure, the blotter method, the agar plate method and the seedling symptom tests. This is true particularly for bacterial and virus diseases. The plants must be grown beyond the seedling stage before symptoms can develop. The seeds are sown in autoclaved soil in suitable containers and submitted to suitable conditions of temperature and humidity, often in a glasshouse or controlled environment cabinet, protected against contamination. A controlled environment chamber, as in the standard procedure developed for lettuce mosaic by Rohloff (1960, 1962), is an ideal of the procedure. A test can be completed in 16–21 days by sowing the seed in peat with 0.5–2.0 g CaCO₃ added per litre and 0.5–1 g complete fertiliser, and incubating at 25°C under constant fluorescent daylight (8×40 W tubes for each 0.6 m², 30 cm above substrate surface). Afanasiev (1956) described a test for barley stripe mosaic that may be conducted in a greenhouse. Barley is sown in flats or polypots, in units of 100 seeds. The first reading is made in the three-leaf stage, the second and final reading about 10 days later. Hampton, Sill and Hansing (1957) established that high light intensities (10,000 foot-candles) and temperatures of 21–30°C are optimum conditions for symptom expression and that symptoms were poor and recorded infection percentage lower at low light intensities (3,000 foot-candles) even at optimum temperatures.