Screening for Disease Resistance in Faba Beans
2.3. Artificial Production of Epiphytotics in Faba Bean Disease Screening Nurseries
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2.3.3. Rust ( Uromyces Fabae ) 2.3.3.1. Urediospore collection U. fabae is an obligate pathogen which cannot grow on artificial media, so spores are harvested directly from the field.Collect heavily rusted faba bean leaves (Fig. 12) from naturally infected plants and tap with a rod over a funnel, the stem of which leads into a storage vial. For large scale inoculation, special spore collectors with a small engine providing suction are available. These spore collectors can be used to collect large quantities of urediospores from the surface of heavily infected leaves. |
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Fig. 12. Rust-infected faba bean leaves.2.3.3.2 Artificial Inoculation
Apply 10-15 ml of a urediospore suspension (400,000 spores per ml) to 12-16 week old faba bean plants in the evening, using a knapsack sprayer. Half fill the tank of the sprayer so that the suspension is disturbed in the tank. This ensures uniform spraying of the spores. Rust nurseries can also be inoculated by following procedure no. 2 for ascochyta blight, using heavily rusted leaves rather than ascochyta blight infected leaves for inoculation. For small-scale screening purposes, a few heavily rusted faba bean plants can be wrapped together in a bundle and brushed against the moistened leaves of the faba bean plants to be evaluated.Whether small- or large- scale artificial inoculations are done, all inoculated plants should be moistened with water and covered with polythene sheets for at least 20-24 hrs. Spraying with water, covering, and uncovering the nursery should continue for up to 3 or 4 days.
Disease readings can be made 2-3 weeks after inoculation.
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2.3.4. Alternaria Spot ( Alternaria tenuis ) A. tenuis can be easily isolated from pieces of infected faba bean leaf on TWA medium. Spores formed in artificial culture are less virulent, so the following procedures are suggested to produce virulent spores. Procedure No.1 2.3.4.1. Preparation of the Inoculum a.
Collect infected dry faba bean leaves (Fig. 13) late in the season
and store in large perforated plastic bags. |
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Fig. 13. Altemaria spot-infected faba bean2.3.4.2. Artificial Inoculation
a. Apply 20 ml of the spore suspension
to faba bean plants after sunset using a knapsack sprayer.
b.
Cover inoculated nurseries immediately with polythene sheets to provide
adequate humidity.
c.
Uncover the plants next morning, spray with water 2-3 times a day, then
cover again at sunset. There is no need to cover the nursery if rainy
humid weather conditions prevail after inoculation.
d.
After 6-8 days, uncover the plants then take disease readings 2-3 weeks
after inoculation.
Procedure No. 2
This procedure is similar to the detached
leaf test and can be used to double-check field results.
a.
Surface sterilize infected faba bean leaves in 0.5% sodium hypochlorite
(10% Clorox) for 1-2 min, plate on TWA medium, and incubate at room temperature
under continuous white light.
b.
After 24-36 hrs remove spore chains with a flamed needle and transfer
them to TWA.
c.
Incubate plates at room temperature under a continuous white light.
d.
After 8 days' incubation, place 5 nits of sterile tap water on the surface
of the plate and scratch colonies gently with a flamed spatula to bring
spores into suspension.
e.
Decant the spore suspension into a beaker containing sterilized tap water,
and add sterile water until an inoculum density of about 400,000 spores/nil
is obtained.
f.
Inoculate healthy faba bean leaves, harvested from 12 week old plants
and placed on the the surfuace of a moistened sponge in a moist chamber,
with drops of the spore suspension.
g.
Incubate at room temperature (22°C) under continuous light.
h.
After 2 weeks, spores on the leaf surface can be brushed off gently in
tap water and used for artificial inoculation.
