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A Plant Disease Primer
by Miranda Smith
Healthy gardens teem with microorganisms: bacteria, fungi, viruses, virus-like organisms, and nematodes. The majority of these are either benign, coexisting in the soil and on plant surfaces with each other and with the plants around them, or they are beneficial, and perform such tasks as breaking down the compost pile into humus, transforming the nitrogen in the air into forms that plants can use, or preying on pest species. Only a small percentage of microorganisms are pathogenic—that is, capable of causing plant diseases. Fungi are the cause of the largest number of plant diseases, while viruses often cause those with the most severe symptoms. Many pathogens cause only cosmetic or superficial damage. But a few of the most virulent kill their hosts outright and can pose a problem in the garden for years to come.
Plants can also develop disorders. These are not brought on by pathogens, but rather by environmental conditions and stresses. Common examples include blossom end rot of tomatoes and peppers, and tip burn of lettuce, both of which are caused by a calcium deficiency in combination with fluctuating moisture conditions and fluctuating light levels, respectively. In most cases, the symptoms of disorders are confusingly similar to those resulting from pathogenic diseases.
Plant Defenses
Unlike animals, plants don't have an immune system or disease-fighting white blood cells. Instead, they have developed other mechanisms to escape or survive the diseases that attack them. Some plants are simply genetically incompatible with, or immune to, certain diseases. A corn plant can't host tobacco mosaic virus (TMV), a common tomato disease, for example, just as tomatoes can't host the corn disease smut. It is also possible that a particular cultivar, variety, or even an individual plant is immune to a disease that attacks other members of its species, generally because of subtle differences in the genetic makeup of the various plants.
Resistance to diseases is another factor that protects plants. Although a resistant plant can be susceptible to a particular disease, it is less likely to become infected by it than a non-resistant plant. In some cases, plants are resistant because of a physical characteristic: for example, their cuticles may be too thick for a particular fungus to penetrate, or their stomata, small pores on the leaves, may be closed at the time of day when environmental conditions stimulate disease spores to germinate. Other types of resistance are biochemical: the plant produces enzymes and proteins that inactivate the pathogen.
A plant is described as tolerant to a particular disease if it can be infected by the disease but is unlikely to be killed or severely disabled by it. Tolerance often results from hypersensitivity: a plant may be so sensitive to a disease organism that the cells infected by the pathogen die very rapidly and the disease organism is isolated and cannot move to other areas of the plant tissue.
Plants can also escape infection. Some diseases, such as early blight of tomatoes, primarily attack plants that are young and growing vigorously, while others, such as late blight of tomatoes, attack those that are old and forming more fruit and seeds than new leaves. So plants that are older when early blight strikes have a better chance of surviving, as do those that are young when late blight attacks. Similarly, some pathogens, such as many that cause powdery mildew, slowly build to damaging populations as the season progresses, while others, such as Pythium, which can cause root rot, are at their most numerous in the early spring. That is to say, it may pay to put off planting crops susceptible to root rot, such as potatoes, until later in spring when the soil has begun to dry, and you may want to take measures to ensure that you can harvest your cucumber crop early, before powdery mildew becomes a problem. When plants do not become infected with a disease simply because of timing, they have "escaped infection."
Disease-causing Organisms
Bacteria
Bacteria cause three types of plant disease symptoms. Some live within a plant's vascular system, the channels through which water and nutrients move from roots to leaves and vice versa. When these organisms become numerous, they are likely to plug up the system, preventing the flow of water and nutrients. Wilts, often beginning on only one side of the plant or even on one branch, are the result.
Other bacteria cause abnormal growth in the cells they have invaded and/or in adjacent cells. These growths, known as galls, may pepper the surface of a leaf or protrude from a stem. They are usually rounded and hard to the touch.
When bacteria kill the cells they infect outright, the results can be rots, internal blights, or spots. Sometimes the spots are small and barely noticeable, but in other cases, cankers, open wounds, or lesions form on bark or soft tissue. When the infection is serious, the bacteria spread and kill the entire leaf, stem, or plant.
Most bacteria are one-celled organisms, although a few are multi-cellular. They reproduce by cell division. Because this reproduction is not sexual, bacteria cannot hybridize. However, that characteristic does not inhibit their ability to evolve; they do exchange genetic information and DNA through plasmid exchange (the exchange of small, DNA-containing elements that exist outside the chromosome), and they mutate relatively easily. Changes in their environment, including ultraviolet light levels and ambient levels of atmospheric gases, as well as subtle changes within their host, can trigger a mutation. This adaptability is responsible for the steadily increasing number of bacteria that cause disease in plants and animals—and the increasing number of bacteria that are immune to antibiotics.
When conditions such as temperature or moisture levels do not suit them, bacteria become dormant. Although many overwinter in the seeds of infected plants or inside insects (the pathogen that causes bacterial blight of cucurbits overwinters in the salivary glands of hibernating cucumber beetles, for example), most overwinter on cankers on dormant plants or in the debris of the plant they infected during the growing season. Though a few species can survive for longer than a year in a dormant state, most die after this time. Consequently, rotating crops so they don't grow in the same area again for two to four years is usually an effective control.
Fungi
Fungi are far more complicated in both structure and reproductive style than bacteria. The fungal body, called a thallus, usually consists of microscopic, threadlike hyphae that spread througout the source of food of the fungus. Collectively, these hyphae are referred to as the mycelium.
The reproductive strategies of different species of fungi vary widely. Some form sclerotia, thickened areas of a hypha, which can break away or overwinter in place and grow into a new organism. Other fungi form spores, tiny bodies that can germinate and grow as seeds do. Some spores are formed asexually; these are usually known as summer spores because they can reproduce as soon as they find appropriate conditions. They tend to die within a few months if they do not find a hospitable environment. Other spores are formed sexually. Many of these are called resting spores because they can remain dormant but viable for months or, if necessary, for many years. Almost all fungal species produce at least two kinds of spores to guarantee their survival, but the fungal rust diseases that require two alternate hosts produce as many as five spore forms over the two years it takes to complete their entire life cycle.
Fungi mutate in the same way that bacteria do; and those that reproduce sexually have the additional advantage of being able to crossbreed. Like bacteria, fungi are capable of evolving rapidly.
Pathogenic, or disease-causing, fungi produce symptoms that are similar to those caused by bacteria: wilts, galls, rots, spots, cankers, and blights, as well as smuts (large growths filled with spores), scabs (roughened skin tissue), and mildews. These last three symptoms are characterized by the visible presence of the hyphae.
Fungi overwinter in plant debris, seeds, or the soil, as spores or sclerotia. Depending on the species, resting spores can remain viable for two to 20 years. It pays to do some research before simply assuming that a crop rotation will protect your plants from infection by a fungus.
Viruses
Viruses that attack plants are just as insidious as those that attack animals. Their physical structure is simple: just a bit of DNA or RNA and a protein covering. Rather than reproducing themselves, viruses stimulate the cell within which they are living to create more viruses. They too evolve quickly in response to changes in their environment.
Viral diseases cause malformations, stunting, and disruption or death of cells containing chlorophyll, which often results in the yellowing of plants (chlorosis). The malformations range from simple leaf curling or puckering to abnormal production of stems or branches, as in the disease called witches' broom. Stunting can be generalized, involving the whole plant, or specific to a particular area. Mosaic diseases are characteristic of viruses that interfere with chlorophyll production. Mosaic-infected leaves are usually mottled with irregularly shaped yellow patches.
Viruses can remain in the host without being active. Many have a fairly narrow band of temperature preference; they are active between 55°F. and 85°F. but not at temperatures above or below this. This characteristic often gives gardeners the false impression that their plants suddenly "got well" from a viral disease.
Plant debris, soils, and gardeners' tools, hands, and shoes can all transport dormant viruses from one host to the next. But viruses remain viable for so long—up to 50 years in some cases—that it is impossible to know for certain how a virus got into a planting. They can also survive the hottest temperatures a compost pile can achieve.
Nematodes
Nematodes are microscopic animals that look like slender, nonsegmented worms. There are thousands of nematode species, but only a few hundred of them cause plant diseases. These are directly responsible for a number of symptoms, including rotting tissues and abnormal growth, but they also cause diseases indirectly. They can carry viruses, injecting them into the plant as they feed. Moreover, when nematodes pierce the plant tissue to move into cells, secondary pathogenic organisms very often take advantage of the openings to invade the plant.
Nematodes reproduce both sexually and parthenogenetically—that is to say, without the benefit of males. They produce eggs, which are released into the soil around a plant or inside the plant itself. Eggs develop into juveniles, the stage at which most nematodes infect plants; juveniles can also remain dormant but viable for months without a host. Nematodes overwinter in soil or plant tissue as eggs, juveniles, or adults. Egg-to-egg life cycles usually range from three weeks to three months.
Spreading Disease
Pathogens move from host to host in a number of ways. When not seed-borne, bacteria are usually carried by animals, insects, moving water, air currents, and infected soil, or spread around the garden on tools and shoes. Bacteria move into plants through natural openings, such as stomata, or through small wounds and bruises. Once inside a susceptible, living host, their environmental needs are met.
Bacteria reproduce within a host for as long as it continues to feed them. If the bacteria are feeding on the surface of a plant or kill enough cells so that the plant tissue breaks open, they are easily moved from one host to another by wind, falling rain, insects, or even gardeners. Those that feed internally usually overwinter on the plant debris or in the soil, ready to be carried to a new host the following spring.
Fungal spores travel the same routes from host to host as bacteria, but differ in that a spore must germinate and grow before it can infect a plant. Germination is primarily dependent upon temperature and humidity levels. Most fungal species require moderate (50°F. to 90°F.) temperatures and very high humidity or a film of water on the leaves. After germination, most hyphae grow and find their way into a plant through natural openings, wounds, and bruises, but some species have an even more direct route. They form a penetration peg, a toughened tip on the end of the first hypha, that bores through the plant's cuticle layer and into the tissue. Other species release a toxin that kills plant cells; once surface cells are dead, they move into the plant's interior, always killing cells in advance of their spread.
While some fungi can feed only on dead material, and some can feed only on living plants, many of the plant pathogens can feed on either dead or living material. This enables them to remain active after their host is dead, as long as temperatures are tolerable for them; once the weather warms in spring, they can start feeding on the host again and produce a new crop of summer spores that are ready to infect new plants.
Viruses are usually transmitted by insect or nematode vectors—organisms that carry and transmit a disease—and gardeners' hands and tools. Vectors generally transmit the disease by inserting it into the plant as they are feeding. Gardeners are more likely to infect a plant by touching it with dirty hands. Viruses are so small that they can be moved into wounds and bruises that are invisible to the naked eye. As long as the host is alive and susceptible to them, their environmental needs are met.
Nematodes that infect plants do so in their larval stage. If they are not carried to new hosts by moving water or on shoes or tools, they swim through the soil solution. However, due to their size, even the speediest nematode rarely travels more than a foot away from its birthplace. Again, the only requirement for infection is that a susceptible host is alive.
Diagnosing Diseases
Symptoms of various plant diseases look so similar that it's often difficult to diagnose them correctly. However, there is a common-sense approach. Begin by trying to rule out the possibility that the plant has a disorder or non-infectious disease—an ailment caused strictly by environmental or cultural conditions—rather than a disease caused by a pathogen.
Disorders include such things as hollow heart of beets and broccoli as a consequence of a boron deficiency, cat-face of tomato as a consequence of low light levels, and sunburn of tomato or pepper as a consequence of excessive sunlight directly hitting the fruit, when the leaf canopy is lost due to pruning or disease.
In general, disorders are fairly uniform throughout a planting or a particular area: if one plant has the problem, the rest do, too, and usually to about the same degree. Diseases are likely to be less generalized. Due to pathogen movement, they may be widespread in the downwind portion of a planting, or along a path where water runs through the garden, but even then, symptoms will vary somewhat from plant to plant. With close enough observation, you may even be able to chart the movement of the pathogen from one plant to the next.
Once you have decided that the problem is caused by an infectious disease, you'll need to try to identify it before you can develop a plan of action. Even if you are the sort of person who dislikes asking for directions when you're lost on the road, it's wise to ask for help in diagnosing a disease. More often than not, seasoned gardeners in your area have seen the disease and can tell you a few things about it. If this fails, get out disease identification books and start looking for likely suspects. Remember that few texts have enough space to list every single plant that a particular disease attacks, but most list the genera or families that host the disease.
Check visual clues too, preferably with a hand lens. Except in the case of spots or internal blights, visual examination with a 10-power lens usually will tell you if the problem is caused by a fungus, bacterium, virus, or nematode. For example, if lesions on the plant are oozing and slimy, chances are that you're looking at hundreds of thousands of tiny bacteria. A mass of threadlike structures is likely to be the mycelium of a fungus, and dry specks that disperse into the air when you breathe on them are probably spores. Leaves that are curled and puckered or have yellowish patches between the veins are an indication that the plant probably has a virus. If the plant looks sickly but you can't see anything above the ground, dig it up. It may have a root-eating pest such as a cabbage root fly maggot or root aphid, but it could also be hosting nematodes. Galls or knots on the root tissue are the sure symptom.
Finally, take a walk around the edges of your property. Look at weedy areas there and in neighbors' yards. It could be that a weed in the same family as your plant is hosting the same disease. Very often, this kind of clue quickly leads to the culprit: a disease that is unrecognizable on your plant might be easy to identify on a weed.
Miranda Smith has been teaching organic agriculture since 1971, and has worked in many regions of the U.S. and several provinces in Canada. She is author or co-author of 11 books about horticulture, including Rodale's Pest And Disease Problem Solver: A Chemical-Free Guide to Keeping Your Garden Healthy.