Screening for Disease Resistance in Faba Beans

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

2.3.5. Stem Nematodes (Ditylenchus dipsaci)

Faba bean disease screening nurseries can easily be artificially inoculated with D. dipsaci using either a larval suspension or heavily infected plant parts. Our findings indicated however, that infected faba bean stems (Fig. 14) are much more efficient than larval suspensions in producing disease epiphytotics. However. large-scale screening of faba beans for resistance to D. dipsaci is best done in heavily infested soil under natural field conditions. For these reasons, the following procedure is suggested:

a. Collect large quantities of D. dipsaci - infected faba bean plants from infested fields early in the season.
b. Cut infected stem parts into small segments (2-3 cm) and keep in plastic bags in the shade.
c. Place seeds from the faba bean disease screening nursery in rows 2m long and leave uncovered.
 d. Place infected stem segments directly on top of the seeds (0.5-1 kg/row), cover with soil, and irrigate immediately. The quantity of infected - stem segments required per row depends on the degree of infection of stem segments.
e. Irrigate the nursery daily until the seeds have germinated and plants are 5-10 cm tall. f. Disease readings and single plant selections should be done just before flowering, with a second reading later in the season when stem symptoms are well developed.
g. Use the same method and site for screening the next season. Within 3 years of continuous soil infestation and faba bean growing, a sick-plot can be obtained for further screening without the addition of infected plant parts.
Fig. 14. Stem nematode lesions on faba bean stems.

2.3.6. Bean Yellow Mosaic Virus (BYMV)

Faba bean disease screening nurseries can be artificially inoculated with BYMV in the greenhouse or in the field. A suspension of virus obtained from infected leaves (Fig. 15) is rubbed over or sprayed onto the surface of leaves of test entries. The infectivity of the viral suspension for artificial inoculation can be assayed on an indicator host such as Clzcnopodium amcirwiticolor before use. Since mixed infections are common, it is very important to verify that the source of inoculum contains BYMV only. This could be done by testing the inoculum on a number of indicator plants and by serology.

The following procedure is suggested for artificial production of disease epiphytotics:

a. Collect infected top leaves from young, vigorously growing, well nourished faba bean plants.
 b. Grind 20 g of infected faba bean leaves thoroughly in a mortar.
 c. Add 100 ml of a 0.01 M phosphate buffer, pH 7.0, and mix well.
 d. Squeeze the juice through a cheesecloth bag into a container.
 e. Add 8 g of 400-600 mesh Carborundum (silicon carliade) or Celit to the juice and mix well.
 f. Rub the inoculum immediately onto faba bean plants using a small piece of cotton or cheesecloth. For large-scale inoculation, a
larger quantity of inoculum can be prepared and sprayed using a knapsack sprayer and about 5 ml of inoculum per plant. The best time for spraying is about 1:00 to 2:00 PM.
 g. Spray the nursery again with water using 15nil/plant immediately after applying the inoculum.
 h. Disease readings can be made twice, at 3 and 8 week intervals after inoculation.

Fig. 15. Symptoms of BYMV (above) and BLRV ( below ) of yellowish haloes around necrotic alternaria spot lesions on two faba bean lines.

The following practices are recommended to increase the efficiency of inoculation:

- Set the engine of the sprayer to its maximum speed and spray from a short distance (10-15 cm from the faba bean leaves). This causes
more wounds on the leaf and facilitates the entry of virus particles into host tissue.
 - Use young (3-6 week old), vigorous, irrigated plants, grown in fertile soil, which are in general, better suited for artificial inoculation.
 - Shade faba bean nurseries for one or two days before inoculation to favor disease development.

2.3.7. Bean Leaf Roll Virus (BLRV)

Bean leaf roll virus (Fig. 15) is a virus which is not mechanically transmitted; it can only be spread by aphids in the circulative manner. BLRV is commonly transmitted by Aphis fabae, A. croccivora, Acyrthosiphon pisum, and Myzus persicae. Other aphid species are also known to transmit BLRV. Therefore, the use of aphids is a must in the artificial production of disease epiphytotics in faba bean screening nurseries. Faba bean plants should be planted in the greenhouse 6-8 weeks before planting the disease screening nursery. These plants should be infested with aphids carrying BLRV (from a source culture) to permit the increase of the inoculum and the multiplication of the aphids.
The aphid population on each faba bean plant can serve to inoculate 20-40 faba bean test plants in the disease screening nursery. Around 10-15 viruliferous aphids per plant are enough to produce 100% BLRV infection. Viruliferous aphids can be harvested over a plastic tray by adding a small amount of talcum powder and gently shaking the plants over the tray. The talcum powder prevents the aphids from sticking to each other and facilitates their addition to the test plants to be evaluated. Artificial inoculation can also be done by placing a faba bean leaf with viruliferous aphids on or under the plant to be inoculated.

 
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