Plant Pathogenic Rust Fungi
Plant Protection Methods
Plant breeding is generally considered to be the best long term method of controlling rusts. Apart from this, the primary methods of controlling rust fungi on crops has been through using eradicative fungicides. Plants which have been selectively bred for resistance still require the use of fungicides, but are generally less likely to become overcome by rust. There are a number of ways to protect plants from fungal infection, and some methods which have not yet been carried out and are potential candidates for future research.
Destroy Alternate Hosts
One control method of rust fungi is to try to destroy their alternate hosts.
In the case of wheat rust this is barberry. This method tries to break the
fungal life cycle when the wheat crop is harvested, as the fungus will have
had less chance to spread to a suitable alternate host. This method can be
coupled with standard plant breeding methods for resistant crops, to increase
its efficacy. However this method of crop protection can be very expensive,
as very large areas of the alternative host plant must be cleared and the
numbers of alternative hosts have to be next to zero before the protection
becomes effective.
Exclude Gamete Vectors
Another method of rust control is the exclusion of insect gamete vectors.
In the case of the Pfunder and Roy experiment, insects attracted to the pseudoflowers
of Euphorbia cyparissias were found to be vital in the reproductive cycle
of Uromyces pisi. This raises the possibility of using a combination of fungicides
and insecticides simultaneously to prevent the infection from spreading and
even reduce the occurrence of the disease. This could be an interesting study
as if the insecticide used is effective there would be few vectors to carry
out the transmission of gametes for the sexual reproduction stage. This reduction
of vectors would be similar in effect to an exclusion of insects by physical
means (like the flower gauze used in their experiment). In this way the rust
incidence could be reduced dramatically, but with potentially far less cost.
Major considerations with this type of research would be the broadness of
pesticide targets, how far outside the main treated area the effect would
reach, and the possibilities of environmental pollution and the potential
for species biodiversity loss.
Select or genetically modify for resistant morphological characteristics
An experiment carried out in 2001 by Carlota Vaz Patto et al. (o) showed that plant morphology can have a profound effect on the pathogenicity of rust fungi. They prepared 88 accessions of Hordeum chilense (wild barley), inoculated them with P. hordei and assessed their rust avoidance. These plants were specifically selected for their varying morphological characteristics and three different Amplified Fragment Length Polymorphism (AFLP) primer combinations. They combined a morphological analysis with AFLP fingerprint data and found that the plants could be separated into three distinct clusters, one of which was highly resistant to the fungal infection. These plants had specific morphological characteristics that made them less susceptible to the disease, such as more erect culms and more stomata per square centimetre on the abaxial leaf surface. In the future genetic modification or selective breeding might lead to new accessions of barley, as well as other crops, which will be more resistant to the rust fungi in this way.
Develop chemical agents that prevent normal appressoria formation
There is a possibility that chemical agents could be used to cause the fungal
appressorium do develop at the wrong time, or in the wrong place, effectively
missing the chance to invade the host by being away from the stomata at the
time of appressorium formation.
Selective Breeding/Genetic modification for greatly increased
or reduced leaf ridge heights
Another alternative is to selectively breed plants that are not good candidates
for appressorium formation. Morphological characteristics such as greatly
increased or reduced leaf ridge height could cause the appressorium to either
differentiate from the germ tube far to early before it comes into contact
with the stomata, or could make the appressorium overshoot the stomatal opening
entirely.