C. Lee Campbell

Management of Plant Diseases

Once the causal agent of a disease has been correctly identified, it is possible to develop plans to manage the disease. During the past 100 years, much research has been conducted on pathogens, diseases, and management methods. Today we can draw on this vast store of knowledge to help us in our efforts to manage plant pests.

Agriculture, Plant Diseases, and Human Affairs

At any given time, on any given day, there is enough food to feed all the people on earth for only about three weeks. Even that small amount of food is unequally distributed, and famine stalks much of the world. One-third of the earth’s population awakes hungry each morning and goes to bed hungry at night. Hunger is a way of life in many developing countries. As stated in a 1980 report to President Carter: “For hundreds of millions of the desperately poor, the outlook for food and other necessities of life will be no better by the year 2000. For many it will be worse Hunger and disease will claim more babies and young children and more of those surviving will be mentally handicapped by childhood malnutrition.” A 1989 survey of leaders in the population field resulted in 100% of the respondents saying they expect today’s population of 5 billion to be more than 10 billion in the next century, which will place such enormous demands on the biosphere, (e.g., arable soils, water supplies, and food) that in much of the world, human existence may be threatened. This dismal and alarming forecast underscores two of the most important problems facing humankind in the twentieth and twenty-first centuries—overpopulation and hunger.

Diseases Caused by Bacteria and Mycoplasmas

When bacteria fill the water-conducting cells of a plant and kill plant tissues by means of toxins, the plant suffers a wilt or blight disease. If the bacteria kill leaf cells, leaf spots may develop. When fruits, tubers, or roots are infected, soft rots may develop, which result in a slimy, foul-smelling mass. Other bacteria stimulate tissue growth to form galls or tumors that resemble cancerous tissues of animals. In addition to killing plant tissues, bacteria can stunt the growth of plants, change plant color from green to brown or black, or cause distortion of leaves, stems, flowers, or fruits.

Diseases Caused by Parasitic Plants

Usually we think of parasites as lower forms of plant life such as bacteria, fungi or nematodes. However, a number of flowering or seed plants are parasites on other plants. Parasitic seed plants vary widely in their dependence on a host. The more independent ones, called half-parasites or hemiparasites, have chlorophyll and roots and can manufacture their own food; but they depend on their host for dissolved minerals and organic substances. Others, such as mistletoe, have chlorophyll but no roots and depend on their host for minerals and water. Some other parasitic seed plants, such as dodder, having neither chlorophyll nor true roots, depend entirely on their host for their existence.

Diseases Caused by Viruses

The symptoms of plant disease caused by different viruses often are quite similar and difficult to distinguish. Symptoms vary widely, depending upon virus strain, host, cultivar, plant age, nutrition, weather, and infection by more than one virus. Accuracy in field diagnosis frequently is improved with knowledge of such factors as the pattern of infected plants in a field and the cultivar being grown. The pattern of symptom distribution, for example, can be of great value in separating mosaic symptoms induced by tobacco mosaic virus (TMV) from similar symptoms caused by some herbicides. Plants in a field with mosaic symptoms induced by a virus seldom display uniform symptoms, whereas plants with mosaic symptoms induced by herbicide injury will appear uniform; that is, all plants in a row or adjacent rows will be the same size, and symptoms will be similar.

Fungal Diseases of Shade and Forest Trees and Decay in Wood

The shade and forest trees of North America are a precious natural resource that humans must protect and treasure. These silent, majestic inhabitants cover glens, glades, and mountain slopes, roof over cathedral groves in hidden valleys, anchor hillsides against the wash of sudden downpours, shelter violets and wild columbine, purify freshets as they rush toward lazy rivers, provide thickets filled with birds’ song and hidden animals, supply lumber and paper, splash vistas with mosaics of orange, yellow, gold, and red in autumn, and etch bare trunks and limbs against the deep mantle of winter snow Trees restore the air for human and animal use, and restore human souls with their beauty, diversity, and nobility.

Diseases Caused by Nematodes

The representative nematode diseases discussed in this chapter are common and can be recognized fairly easily. It should be remembered that it is easy to mistake nematode diseases for those caused by some other organisms, and vice versa. Merely finding nematodes in diseased plant tissue or the soil is not conclusive evidence that they are the cause of the trouble. Nonparasitic types of nematodes often are found in great numbers in decaying plants, and the soil always contains a variety of free-living nematodes. One should always obtain positive identification before starting expensive or troublesome management measures. On the other hand, nematodes should always be considered as a possible cause of plant diseases when root systems are galled, shortened, or reduced by rotting; the stems are shortened and thickened, and the leaves do not grow normally; or some other abnormal growth is noted. Important genera of plant-parasitic nematodes are listed in Table 10.1.

Chemical Management of Plant Diseases

Some people think that chemical management of plant diseases was invented just a few years ago, but it was 2000 years ago that the Greek poet Homer, author of The Illiad and The Odyssey, wrote about the “pest averting sulfur with its property of divine and purifying fumigation.” Sulfur is still used today as a pesticide.

Diseases Caused by Soilborne Fungi

Soilborne fungi cause some of the most widespread and serious plant diseases. Spores or mycelium of many of these fungi can overwinter or survive adverse conditions in soil or on plant debris, and once an area has become infested with soilborne fungi, it is generally difficult to get rid of them. Often root diseases caused by soilborne fungi may not be noticed until extensive damage has been done, because many of the early symptoms occur only on roots. The representative diseases discussed in this chapter are summarized in Table 12.1.

Causes of Plant Diseases

Infectious plant diseases are caused by living (biotic) agents, or pathogens. These pathogens can be spread from an infected plant or plant debris to a healthy plant. Microorganisms that cause plant diseases include nematodes, fungi, bacteria, and mycoplasmas. We also classify viruses and viroids as biotic agents because they must have living cells for reproduction and are composed of nucleic acid and protein. Some higher plants that produce seeds are parasitic on other plants and are considered to be pathogens. Each type of plant pathogen is discussed briefly in this chapter. More detailed information is given in the chapters dealing with the individual diseases. The relative sizes of various types of pathogens are illustrated in Fig. 2.1.