Types of plant pathogens

This post is meant to simplify plant diseases and

their control. I hope I don’t offend any plant pathologist

out there. My father was a plant pathologist, and he might

have cringed at some of my generalizations, but here goes.

The three major classifications of plant diseases are

those caused by fungi, bacteria, and viruses. Yes, there

are other classifications of pathogens, but I’ll not

discuss them here. Fungi are usually multicellular,

bacteria are single celled, and viruses are not a true

living organism at all, as they cannot reproduce without a

living host.

We’ll begin with fungus diseases. Some fungus diseases

are systemic, which means that the fungal mycelium goes

inside the plant. An example of this is silverleaf disease

in apples. There is no effective way to treat this disease

outbreak other than removing and burning the affected

trees.

Most fungus diseases aren’t as difficult to control.

Part of the reason involves knowing a little about the

disease triangle. Three things are necessary for a disease

to occur. The environment must be suitable, there must be a

pathogen and the plants must be susceptible to that

pathogen. If the environment is not suitable for growth of

the fungus, disease will be avoided. Otherwise, treatment

will be necessary.

However, if you know that, you can treat early and

avoid much of the problem. Should you fail to do this, you

often can treat infected plants and solve the problem.

Removing infected tissue or treating with a fungicide are

treatment options. Apple scab is a fungus disease that can

be controlled with regular spraying.

Fungi don’t reproduce as fast as bacteria, which leads

us to bacteria. They are single celled primitive

microorganisms called prokaryotes. This means that their

cells have no nuclei. Under favorable circumstances, they

can reproduce at an alarming rate. That’s why plants can

look healthy one day and nearly dead the next.

We all have experienced bacterial infections and have

taken antibiotics (bactericides). We usually take them two

or three times a day for ten days or so. This kind of

treatment regimen is impractical for plants. The only

effective ways to control bacterial diseases in plants are

to remove the infected plant to save the rest or plant

cultivars resistant to the disease.

A common bacterial disease you might see in a garden

is bacterial wilt, which infects cucurbits and tomatoes.

Removing insect vectors like cucumber beetles can lessen

the problem. So can improving air circulation and not

overwatering.

Fire blight in pears has made it virtually impossible

to grow bartlett pears in our climate. Citrus greening

disease and citrus canker have crippled the Florida citrus

industry in recent years.

Viruses are perhaps the most difficult pathogen type

to control. Viruses attach themselves to the cell of a host

and reprogram that cell to produce more viruses. These tiny

pathogens are nothing more than a piece of DNA or RNA

covered with a protein coat. There is basically no chemical

control for them. Removing infected plants is about the

only thing you can do once plants become infected.

Probably the most effective way for plants to avoid

being damaged is to reduce the stress on them. Proper

nutrition and water management are important but investing

in resistant varieties is the most effective way to avoid

losses.

I’m only scratching the surface here. Plant pathology

is a complicated science, and I have made some sweeping

generalizations. Not only did I not even mention all the

pathogen types, but I also didn’t even discuss abiotic

factors like mineral deficiencies, which are very common.