The Basics of Plant Diseases

In my experience, many people who grow plants, whether for food, pleasure or profit, often see problems with their plants showing poor growth, blemishes on leaves, dead or dying branches, or outright plant death. When that happens, the following questions come to mind:

• What is causing the problem?
• Where did it come from?
• How do I get rid of it?
• How can I prevent it from happening again?

As a plant pathologist for many moons, I have asked these questions privately for my own plants, grown for food and landscape beauty, and professionally for commercial nurseries and farmers. As gardeners, both outdoors and indoors, I judge that most of you have asked these questions, too.

In this series of articles, I will present the basics of plant diseases in hopes that you can better address any disease problems that may crop up with your plants. This first installment will focus on how to determine whether the problem you observe is either an infectious disease or a non-infectious problem brought on by some growth factor in your soil or growing environment.

The Disease Triangle

For an infectious disease to occur, three fundamental elements are required: a susceptible plant, a pathogen capable of causing disease and a favorable environment. If any of these elements are missing, no infectious disease occurs. On the other hand, if no pathogenic agent is present, but the plants exhibit symptoms such as poor growth or yellow or necrotic leaves, and those symptoms occur on most of the plants, then a non-infectious disease is likely.

Signs and Symptoms of a Plant Disease

When you suspect something is wrong with the way your plants are growing, the first step in diagnosing the problem is to characterize the symptoms you see.

Symptoms may be localized, such as leaf spots, or systemic, such as stunting or off-color foliage. In my experience, stunting can be a general response to a weakened root system that is not functioning to capacity. Examination of the root system may reveal that many of the roots are brown and dead-looking instead of white and robust.

Thus, the problem may be localized in the roots but causing stunting of the above-ground part of the plant. Another thing to consider is whether all the plants exhibit the same symptoms or only a few here and there. If the latter is the case, I usually try to determine what is different between the sick and the healthy plants.

If the symptoms you observe are on young seedlings that came up and then fell over (called damping-off), or the seeds never germinated in the first place (called pre-emergence damping-off), you should see that as a sign of an infectious agent that attacks seeds or young seedlings. If the plant suddenly wilted and died after growing for a while, then you should consider other so-called wilt or root rot diseases.

Sometimes the foliage exhibits some changes in coloration or some twisting or other deformation of the foliage. The color pattern may be a mosaic of green and yellow, or some other symptom, such as a leaf mottling or streaking, often occurring with most of the plants. This suggests the possibility that the causal agent is inside and throughout the plant, and that the disease is caused by a virus. Virus disease symptoms may be confused with those caused by nutrient deficiencies.

Where plants are close to each other, such as in lawns, the disease may occur on multiple plants and appear to be spreading from one plant to its neighbors. In examples involving turf grass, the plants are very close and symptoms will be expressed by the group of plants.

When individual plants suddenly wilt in spite of the soil being adequately irrigated, one might suspect that the plant contracted a disease from the soil. The wilt diseases, caused by either fungi or bacteria, enter the roots and grow into the vascular system—the water and nutrient-conducting cells.

The pathogen can grow up the vascular cells and build a blockage that impairs movement of water up from the soil into the plant. Generally, individual plants exhibit wilt symptoms unless the pathogen has been well dispersed in the planting soil and infects multiple plants. Such wilt diseases are unlikely to occur in soilless media where the pathogen would not be present. In a few plants that we know of, the pathogen is seed-borne, including spinach and basil.

Diagnosing Plant Problems from Symptoms and More

Diagnosing a plant problem may be done based on symptoms alone in some cases. Take the fusarium wilt disease, for example: a single tomato plant in your garden suddenly wilts in spite of adequate irrigation.

Sometimes the wilting occurs on only one side of the plant and if you slice downward into the stem of that branch, you may see brown streaks, which could indicate a vascular wilt disease.

A cross section of the stem might also show the browning in spots. Most tomato plants from nurseries have built-in resistance to fusarium wilt, but you may be growing an older variety that does not have that resistance. Sending your wilted tomato to a diagnostic lab where the pathogen can be isolated and identified would be the clincher in diagnosis.

What if your plant wilts, but there are no tell-tale symptoms in the stems that would point to fusarium wilt? Check the roots and see if they look healthy or diseased. If enough of the roots look rotten, that may be enough impaired root function to cause the plant to wilt when water becomes limited.

If the roots look good, then the cause may be due to some cultural problem, such as too little water. On a side note, it is well known that plants that have mycorrhizal fungi in their roots can withstand soil drought much better than plants without.

Root examination might also reveal that the roots are not so brown, but they have swollen areas or knots on them. That would be a tell-tale sign the plant has root knot nematode infection. Nematodes are tiny worms that infect the roots to complete their life cycle, and in so doing, disrupt the root function enough to cause stunting and even wilt.

Take another example like tomato leaf spot. Your tomato plants looked good in the early part of the growing season, but then spots started to appear on the leaves. You are sure your overhead sprinkler system is working to keep the plants well watered, but the disease continues to spread to more leaves as the plants grow—and it is spreading fast.

Your plants could have fungal leaf spot caused by the fungus septoria. When leaves become infected, the fungus produces many new spores that splash onto other leaves when the sprinklers come on. The spores germinate, penetrate the leaf tissue and begin to kill the cells.

Eventually the tomato fruit will also have those spots. This disease is on a rampage, and only two things will slow it down. First of all, change your irrigation system from overhead sprinklers, as standing water on the leaves allows the spores to germinate and begin infection.

Keeping leaves dry during the day will stop those new infections. Next year, or next time you start a tomato plant, avoid overhead irrigation. Secondly, clean up all of the fallen, infected leaves on the ground as they will be the source of next year’s infection.

Some foliar diseases are obvious when you see symptoms of infected leaves compared to non-infected leaves. For example, some maple trees grown in landscapes are susceptible to powdery mildew. This fungus disease causes superficial infections on leaves, but the tell-tale symptom is the production of white, powdery masses of spores on the infections.

Some infected landscape plants, however, lack the abundant sporulation that would distinguish the problem as powdery mildew. The powdery mildew pathogen, and some other diseases caused by fungi, such as rusts and downey mildew, cannot be cultured in the lab, so diagnosis has to be done by symptoms alone and observing spore production under the microscope.

So, what if your plants do not exhibit any of the tell-tale symptoms discussed above? Perhaps most of your plants show the same stunting or yellowing discoloration, or maybe the plants are still green but are not growing as well as you would like.

Assuming there have been no environmental extremes, one should consider irrigation and watering practices. Over-watering can damage the roots enough to impair their function; under-watering can simply cause the plant to experience drought stress. During drought stress periods, plants are not able to function well, and growth can be slowed as a result.

What about fertilization? Too little fertility in the soils could deprive the plant of needed nutrients. For example, the needed phosphorus in many soils is unavailable because it is bound to clay particles or has formed an insoluble precipitate.

That means phosphorus is immobile in the soil. Once again, the solution is to add more fertilizer to meet your plants’ needs, or inoculate your plants with mycorrhizal fungi at planting. These symbiotic fungi colonize roots and help the plant acquire phosphorus from more distant places and pipe it back to the plant.

(Check out Basics of Plant Diseases: Part Two, and Basics of Plant Diseases: Part Three)