Screening for Disease Resistance in Faba Beans

Faba bean disease screening nurseries should normally be established in humid areas where environmental conditions favor maximum disease development. Depending on seed availability, germplasm lines are grown in rows 1 or 2 m long and 30-50 cm apart. Crowding plants together favors disease development, although clustering plants too much may yield tall stand and cause lodging. Therefore, a moderate planting density
is suggested. To optimize plant growth and favor the expression of maximum genetic capabilities by the host plant, good agronomic practices should be applied.

Planting date is an important factor in disease development. In general, chocolate spot, ascochyta blight, and stem nematodes are favored by early planting, while rust and powdery mildew occur in late planted fields. However, the exact planting dates that favor specific disease, depend on local conditions. At Lattakia in Syria, chocolate spot, ascochyta blight, and stem nematode nurseries are normally planted in October and rust nurseries are planted in November.

It is regular , practice to include a local disease-susceptible check cultivar in disease screening nurseries. Local susceptible cultivars should be grown as frequently as every third row to help spread the disease and develop a uniform disease pattern throughout the nursery. Local susceptible cultivars also serve as a standard against which disease rating is done, when different lines in the nursery are evaluated for disease resistance. It is advisable, but not absolutely necessary, to plant spreader rows first, early in the season, wait for 2 or 3 weeks, and then plant test lines. This practise induces early disease development on local checks which can later serve to spread the inoculum to adjacent test entries.

2.3. Artificial Production of Epiphytotics in Faba Bean Disease Screening Nurseries

Specific procedures for specific diseases are shown below. Most of these are drawn from experimental work involving certain parameters which can be artificially manipulated to produce disease epiphytotics. Best results are obtained when artificial inoculations are done on cloudy rainy days, particularly at sunset.

2.3.1. Chocolate Spot ( B.Fabae )

Procedure No.1 for preparation of the inoculum

This procedure is suggested where trained pathologists and laboratory facilities are available.

2.3.1.1. Isolation of B.Fabae

a. Collect black sclerotia (Fig.5) from cultures of B. fabae kept in the refrigerator from the previous season or collect infected faba bean leaves or stems having the aggressive stage of chocolate spot (Fig. 6) (dark brown blackish lesions) from plants grown during the current season.
b. Surface sterilize the sclerotia, or infected tissues taken from the advancing margin of a lesion, in a 0.5% sodium hypochlorite solution (10% Clorox) for 1-2 min, plate on FDA medium, and incubate at room temperature (20-25°C).

2.3.1.2 Propagation and Harvest of B.Fabae Spores

A. On Artificial Culture Medium (FDA)

a. Place 5-10 mls of sterile tap water on the surface of pure colonies in petri dishes.
b.Rotate the plates gently until spores are released into the water.
c. Transfer this spore suspension to plates and spread the suspension over the entire agar surface by shaking the plate gently. This ensures uniform growth throughout the dish.
d. Stack plates on top of each other in groups of 10 and incubate at room temperature for 1-3 days, until mycelial growth can just be seen on the agar surface.

e. Arrange the plates side by side in one layer on a bench and expose to an alternating cycle of 12 hrs light from a 40W fluorescent tube and 12 hrs darkness. After 3 days stop the light treatment, and use cultures when they are 12 days old. (Over-exposure to light, particularly UV light, may affect the virulence of B. fabae).

f. To harvest the spores, place 10-15 mls of tap water on the surface of the 12 day old culture and rub gently with a small brush.

g. Dilute with sterile tap water untill a spore suspension containing about 400,000-500,000 spores/ml is obtained. (Average number of spores is determined from counting spores in 10 microscopic field and converting the average into spores/ml ) .

B. On Faba bean Leaves

  a. Place 5-10 mls of sterilized tap water on the surface of pure colonies in petri dishes.
  b.Rotate the plates or scratch the surface of the culture gently until spores are brought into suspension.
  c. Decant the spore suspension into a beaker containing sterilized tap water and add more water until a spore suspension containing about 400,000 spores/ml is obtained.
  d. Inoculate healthy faba bean leaves, harvested from 12 week old plants, with drops of spore suspension and place on the surface of a moistened sponge in a moist chamber. Moist chambers with dimensions of 40 X 80 X 6 cm covered with polythene sheets are used successfully at Lattakia (Fig. 8).

  e. Incubate the chambers at room temperature (22°C) under an alternating cycle of 12 hrs white light and 12 hrs darkness.

  f. . After 2 weeks, spores grown from leaf surfaces (Fig. 9) in moist chambers can be brushed off gently in tap water and used for artificial inoculation.

Note that the above procedure is very similar to the detached leaf test, where leaves of several faba bean genotypes can easilybe evaluated for chocolate spot resistance ( Fig. 8)