EGARDENING   Friday, November 02, 2007

Resources

Fighting Fungal Disease: IPM Part 2

Black spot, powdery mildew, downy mildew, botrytis, rust ... you've probably seen these disfiguring diseases in your gardens. They are all caused by a diverse group of parasitic fungi. Some are biotrophic (from the Greek: bio - life, trophy - feeding) fungi; others are necrotrophic (from the Greek: necro - death) fungi. As you may guess, necrotrophic fungi are the more lethal.

Necrotrophic parasites destroy plant tissue. Botrytis cinerea which causes the common 'grey mould' on strawberries is an example of a fungus that essentially rots its host. While some of these pathogenic fungi release enzymes that dissolve plant tissue, others block the water-conducting xylem vessels of a plant causing the plant to wilt and die. They literally feed on the spoils.

Rusts, spots and mildews are typically biotrophic fungi. They invade only a few of the host's cells, creating nutrient-absorbing structures termed haustoria. These organisms prefer to keep the host alive as a long-term source of food.

That is not to say that biotrophic fungi can't have enormous impact. As Gail Shumann explains in her book Plant Diseases: Their Biology and Social Impact, the British are known as tea-drinkers today, because of biotrophic rust fungi that devastated coffee production over a century ago. The story goes like this. During the early 1800s, coffee was the beverage of choice in England. Millions of pounds of coffee were being imported from expansive coffee groves in what was then Ceylon. However, around 1870 an epidemic of rust fungi began spreading through the vast the coffee plantations and within decades coffee became so scarce and expensive, that the English were forced to drink tea.

So, all pathogenic fungi can be a problem. How do we fight such fungi?

In 1880, the British government sent Harry Marshall Ward to Ceylon to try and fight the coffee disease. The young Ward was a crytogamist - an arcane term used to describe someone who studied organisms that reproduced with "hidden seeds" or spores. He had just completed studies in Germany with Anton De Bary, who had discovered the cause of the Irish potato blight. Ward soon identified a fungi, Hemileia vastatrix, as the source of the coffee disease. He observed that after fungal spores landed on coffee leaves, they would send filaments, termed hyphae, into the leaf tissue and begin to feed. While there were few fungicides known at the time, Ward did recognize that chemicals could provide some control if they were applied before spores could germinate and penetrate into leaves. However, Ward also realized that the disease had spread too widely to be easily stopped. In his reports to the Colonial government, he explained that the establishment of monocultural coffee farming was greatly to blame for the epidemic. Coffee trees in the wild seemed little affected by rust disease. To stem the tide of the fungal disease, he suggested interplanting, improving cultivation practices, feeding the trees with manure and carefully pruning and tending them. Growers found it easier to replace the coffee with tea and coffee growing was shifted to the new world.

While Ward did not solve the fungal problem, his advice still holds today. The best way to deal with fungi is to first avoid it, by planting a diverse community of plants well suited to the growing conditions and second, to provide the proper cultural support. Of course, since Millardet introduced the first modern fungicide (the lime and copper sulfate, Bordeaux mixture) in 1885, we now have an arsenal of fungicides to help keep fungi at bay. However, it is best to try and create a garden rich in biota that will naturally reduce disease.

Next time, I will describe a typical Integrated Pest Management regime for some common garden fungal problems.

Resources:

General Information on Pathogenic Fungi

Information on Coffee disease, Hemileia vastatrix, and Harry Marshall Ward