By ecology is meant the influence of such environmental factors as climate, weather, soil and fertilizers, on the disease, its severity, epidemic occurrence, and the like. These factors may influence the severity of the disease by their effect on either the pathogen or the host, or both. For example, most fungous parasites require the presence of water on the host plant in which their spores may germinate, hence severe epidemics of such diseases as potato-blight, apple-scab, brown-rot of stone fruits and black-rot of grapes usually appear in wet seasons. Moreover, the attacking pathogen is especially favored by wet weather at certain seasons or periods in its development, especially the infection period. Continued spring rains about blossoming time favor apple-scab and peach leaf-curl. Late summer rains bring with them epidemics of late blight of potatoes, brown-rot of peaches or late infections of apple-scab. Frequent or continuous rains during June and July in grape regions are usually accompanied by severe attacks of the black-rot pathogen. The relation of rainfall to the pathogen explains why, when there has been a severe epidemic the previous season, the crop may escape if the following season be dry.

There is ever a critical period in the development of the pathogen, usually when it is passing from its resting or winter stage to the active vegetative period of the growing season. Moisture and temperature conditions at such periods largely determine whether the disease will be epidemic or not. Of course the necessary abundance of spores to be disseminated is an evident necessity. Favorable weather alone cannot bring on disease as the grower too often believes.

The absence of rains at certain stages in their development is for other pathogens equally essential. The loose smuts of cereals afford good examples. Their spores are powdery and wind-borne and if rains fall when they are being disseminated, they are washed to the ground and perish instead of finding their way into the open blossoms of their host. Thus, clear sunny weather during the blossoming period of wheat and oats one season usually means a more or less severe epidemic of smuts the next, while rains at this time, even though there be an abundance of the disease, may mean a clean crop the following year.

Disease of cucumber leaf. The dying margin indicates that the trouble is due to some interference with the food supply. (X 1/2)

Fig. 1284. Disease of cucumber leaf. The dying margin indicates that the trouble is due to some interference with the food supply. (X 1/2)

On the other hand, weather conditions may determine the severity or absence of certain diseases by its effect on the host. Long-continued cold rainy weather in the spring, especially following a warm spell, results in a slow succulent growth of the developing peach leaves, rendering them especially susceptible to the attacks of the leaf-curl pathogen.

The application of certain fertilizers to the soil is known to have a direct effect, either favorable or unfavorable, on different pathogens. The application of lime or of manure to the soil tends greatly to increase the scab of potatoes planted thereon; while, on the other hand, liming the soil prevents infection of cabbage and cauliflower by the club-root pathogen. Lime likewise favors the development of the root-rot of tobacco and ginseng caused by Thielavia basicola, while applications of acid phosphate tend to prevent infection by this pathogen. The effect of fertilizers on the susceptibility of the host has also been shown to be marked in certain cases. Barley, when fertilized with nitrogenous manures, becomes very susceptible to attacks of the mildew Erysiphe graminis. Certain varieties of wheat have been observed in Denmark to suffer severely from attacks of the rust Puccinia glumarum only when nitrogenous manures are applied. Excessive applications of barnyard manure to greenhouse cucumbers often cause a physiological disease, the symptoms of which are a curling, and dying of the margins of the leaves, accompanied by marked chlorosis or yellowing.

Fertilizers or late continued cultivation of pear trees, by lengthening the period of active twig-growth, favor fire-blight, the bacteria of which infect only tender actively growing tissues.

Peaches of last year's crop still hanging on the tree, attacked by monilia. The branch is dead from the effects of the fungus. (X 1/2)

Fig. 1285. Peaches of last year's crop still hanging on the tree, attacked by monilia. The branch is dead from the effects of the fungus. (X 1/2)