Screening for Disease Resistance in Faba Beans
Seed testing can easily be done in the laboratory to detect seed-borne pathogenic fungi and nematodes.
Seed Testing for Fungal Pathogens
This procedure is good for detecting A. Fabae. B. fabae, A. tenuis, and others (Johnson and Curl 1972).
a. Surface sterilize infected seeds
in a 0.5% sodium hypochlorite solution (10% Clorox) for 2-4 minutes.
b. Plate on FDA or PDA medium and
incubate at 22°C.
c. Expose plates to an alternating
cycle of 12 hrs near ultraviolet light and 12 hrs white fluorescent light.
If ultraviolet light is not available then expose cultures to an alternating
cycle of 12 lirs darkness and 12 hrs light from a 40 W fluorescent tube,
to enhance sporulation (Fig. 7).
d. Examine plates after 8-10 days'
incubation.
Seed Testing for Stemm Nematodes
Faba bean pods at the bottom of the plant frequently become infected with the stem nematode Ditylenchus dipsaci. Heavily infested seeds contain many dried, fourth-stage larvae of the stem nematode, particularly in the necrotic lesions that are frequently seen on both sides of the radicle on the cotyledons.In Syria, live fourth-stage larvae of D. dipsaci were extracted from dry seeds, 3 years after harvesting. The following procedure (Soutliy 1970) is suggested for nematode assay in faba bean seeds:
THE Seinhorst Mist Extraction Technique
a. Place 100 g of seeds on a layer of cheesecloth, in a 60 mesh sieve (Fig. 3).
Fig. 3. Seinhorst mist extraction apparatus for nematode assay in plant parts and soil.
Water mist, A, plant part or soil B, cheesecloth C, sieve D, funnel E, and beaker F.
c. Spray the seeds with a continuous fine mist of tap water at a rate of 41/hr.Heating the water to 20°C improves extraction especially for those larvae underneath the seed testa.
d. After 48 hrs of spraying, remove the beaker and allow the leachates to settle for 5 hrs, then decant carefully, leaving 40-50 mls of extract in the bottom of the beaker.
e. Pour this extract (containing larvae) into a counting dish and examine under a microscope at about 25 X magnification.
f. Measure the length of at least 15 males and 15 females to determine whether they belong to the giant race (1.5 - 1.7 mm) or to the Oat race (1.2 - 1.4 mm).
g. This technique may also be used to extract active plant parasitic nematode larvae from soil or infected plant tissue.
There are several methods to extract plant parasitic nematodes from soil or infected plant parts. The following methods are quick and easy (4).
The Bareman Technique (Modified)
a.
Place a thin layer (1-2 cm) of soil, plant tissue cut into small pieces,
or seeds on a cheesecloth in a 40 or 60 mesh sieve (Fig. 4).
b. Submerge the sieve in tap water
in a shallow dish. The sieve should be supported so that it stays 1 cm
above the bottom of the dish.
c. After 24 hrs, remove the sieve,
pour the water from the dish into a glass cylinder, and allow to settle
for 5 hrs, then decant carefully leaving 50 mls of concentrated nematode
suspension at the bottom of the cylinder.
d. . Pour this suspension into a counting
dish and examine under a microscope at about 25 X magnification.
The disadvantage of the above technique is that there is poor oxygenation in the suspension which makes the larvae motionless.
 |
||
 |
The Seinhorst Mist Extraction Technique
This technique employs the same procedure as mentioned for nematode extraction from seeds (Fig. 3).