The Effect of Weather on Diseases

Paul R. Miller.

Three things must happen at exactly the same time if an infectious plant disease is to occur. One, a susceptible plant must be in a vulnerable state. Two, the parasite that causes the disease must be in an infective stage. Three, environmental conditions must be favorable for disease development.

The environment of a plant consists of the air around it and the soil in which it grows. The environment of a parasite alternates between the body of its host and either air or soil, according to whether it attacks plants above or below ground.

The aerial environment actually is the weather. It is made up of light, temperature, snow, rain, atmospheric humidity, dew, cloudiness, sunshine, wind, air currents, evaporation, and atmospheric pressure. Perhaps each element of weather affects the occurrence of disease in some way, but temperature and moisture apparently are the limiting factors for most diseases.

The amount of heat and moisture available in the soil environment depends on weather (or on irrigation), together with the ability of individual soil types to absorb and retain heat and moisture.

The surface of the soil affects the air in contact with it and a short distance above it. The state of the atmosphere near the ground, therefore, is different from that higher up. Depending on topography, the direction of exposure, the color, type, and moisture content of the soil, the amount and kind of plant cover, and other circumstances, entirely different environments microclimates can exist close to each other. As all plants live wholly or partly within this lowest zone, the weather in it affects them and their diseases.

The weather of any one region fluctuates over a rather definite range and averages up into a characteristic climate. For any given period, however, weather may not conform to the regional climate at all. That is why forecasts are indispensable to any undertaking that is greatly influenced by weather conditions. If weather progressed exactly according to climatic specifications, forecasts would not be needed. The salve thing is true of many plant diseases.

The effect of weather on a plant disease is a consequence of its action on the susceptible plant (the host); on the parasitic organism (the pathogen that causes the disease); and on the relation between host and parasite.

One of the most noticeable things about plant diseases is that some of them occur wherever the host plant is grown and that others are restricted to certain parts of the host territory. Again, a disease that is more or less constantly present over a wide area may always be destructive in some locations but normally insignificant in others. That is because of the action of climate.

Climate is the average weather of a locality or region. It includes the seasonal progress of the weather as well as the extremes. For a season, a year, or a series of years, weather may vary in one respect or another from the "normal" for the climate of which it is a part, but over a long period climate is as definite a feature of a region as its soils, rivers, and forests. In fact, climate is largely responsible for the regional characteristics expressed in the landscape.

So it is with a plant disease. The total effect of weather upon it is summed up in its geographical distribution. We cannot always be sure of the precise explanation, but we know that most diseases flourish best in certain kinds of climate. Just as with the plants that they attack, some pathogens prefer cool regions and others are restricted to warmer areas; some require a great deal of moisture and others get along with less.

Thus the climate of a region determines the crops that can profitably be grown there and also the diseases to which the crops are subject. To put it the other way around: Given the presence of susceptible plants, the area of occurrence of a plant disease depends primarily on the climate. The outside boundaries of distribution generally are the extremes of hot and cold and wet and dry that the parasite can endure.

Within a range of conditions that permit its existence, a plant disease may be an insignificant or an important factor in crop production, depending on how exactly the local or seasonal conditions fit the requirements for its development and spread. Also, within this range, some factor other than weather may assume the decisive role susceptibility of host varieties, for example, or the type or reaction of soil, or cultural management.

A few examples illustrate some ways in which climate is responsible for the distribution and importance of plant disease.

Apple scab, caused by the fungus Venturia inaequalis, occurs almost everywhere apples are grown. It is absent or unimportant only in hot or very dry regions. In areas with cool, rainy springs, it becomes a limiting factor and constant attention is required to control it. In the Northeastern and North Central States, climate is especially favorable to scab, which is by far the most important apple disease there.

During a series of surveys to determine the incidence of cotton seedling diseases and boll rots, technicians discovered that the occurrence and nonoccurrence of anthracnose caused by Glomerella gossypii were definitely separated by a line running through eastern Texas and Oklahoma. The line nearly coincided with the boundary between below 10 and over 10 inches of average summer rainfall. Anthracnose is a constant and important factor under high summer rainfall in the eastern part of the Cotton Belt, but is practically nonexistent in the western part, which has lower rainfall.

Onion plants are susceptible to infection by the soil-borne spores of the smut fungus, Urocystis cepulae, for only a very short period in the seedling stage before emergence. Warm soils hasten the growth of the seedling and allow it to escape infection. In the South, onion seed mostly is planted in the fall and germinates in warm soil. Smut is scarcely known in the South, therefore, although it must have been introduced time after time with onion sets. By contrast, the disease is important in old established onion-growing districts from Kentucky northward, and is still spreading in northern sections.

Some diseases occur widely because the time of year at which susceptible plants are grown in different regions favors their development.

Potato late blight, caused by the fungus Phytophthora infestans, which requires cool and moist weather, is one such disease. Most races of the fungus cannot survive high summer temperatures. The disease occurs in the South because potatoes are a winter and spring crop there and because the organism is reintroduced with infected seed tubers each season.

Climate can constitute an effective barrier against the advance of a plant disease from one region to another. For instance, the Great Plains seems to act as a barrier against curly top of sugar beets. The virus that causes the disease affects many other kinds of plants, including tomato, cucurbits, and beans. The disease occurs in the intermountain region and westward from north to south, but not farther east than the western edge of the Great Plains. Its failure to spread eastward may be due to some climatic relation of the insect that carries the virus, the beet leafhopper Circulifer teneffils.

Several important soil-borne organisms cannot stand low temperatures for long. Among them are the bacterium Xanthomonas solanacearum, which causes Granville wilt of tobacco and attacks many other kinds of plants; Sclerotium rolfsii, the cause of southern blight; Macrophomina phaseoli, which causes charcoal root rot or ashy stem blight; and Phymatotrichum omnivorum, to which the so-called Texas root rot of cotton is due. All attack many kinds of plants. All are practically restricted to the southern part of the country. The cotton root rot fungus is further limited to the southwestern region, where it is native to certain types of soil. The first three organisms have been carried by various means to more northern areas but are not likely to become constantly present there because of their temperature limitations.

A soil-borne organism that prefers cool temperatures but can endure warm periods -can achieve a much wider permanent distribution. Sclerotinia sclerotiorum, another fungus with a long list of hosts, occurs in nearly all parts of the country. In the South it is active during cool weather.

A DISEASE and the pathogen that causes it pass through a series of stages: Carry-over of the pathogen from one season to another; primary (first-season) infection of the host by the pathogen; growth of the pathogen within the tissues of the host; reproduction of the pathogen and secondary spread to new host plants; and finally production of the carry-over stage of the pathogen again.

At each these stages temperature and moisture must be within a range (depending on the particular disease) that permits the process to continue. A cumulation of favorable stages induces severe disease. Unfavorable conditions at any of the stages retard development of the disease or may stop it entirely.

Either temperature or moisture can be decisive in the initiation, development, and spread of disease. If one is constantly favorable, the other becomes the limiting factor. If both temperature and moisture fluctuate, both must be favorable at critical times. If both are constantly favorable, the disease becomes serious.

The amount of inoculum (infective stage of the pathogen) that will be available to start new infections at the beginning of a season depends on the extent of infection at the end of the preceding season and on how well the pathogen can overwinter. Abundant overwintering and favorable conditions for infection in the early part of the season lead to heavy primary infection; if conditions remain favorable, the advantage of the early start is maintained throughout the growing period of the crop.

Some pathogens overwinter in the tissues of their hosts for example, the potato late blight fungus in infected tubers, the peach bacterial spot organism, Xanthomonas pruni, in cankers, and the numerous seed-borne pathogens in seeds. Soil fungi and bacteria may overwinter on host debris. Many fungi, for instance many of the cereal smuts, produce a particular kind of spore that is able to survive the winter temperatures. Some of the spores will not germinate unless they have been subjected to low temperatures. Sometimes alternating warm and cool or wet and dry periods are required for spore germination or to complete the development of primary inoculum.

Some fungi, in addition to or instead of spores, develop a different kind of special organ to carry them over unfavorable periods of various kinds; these are sclerotic.