The further examination of a disease problem from the point of view of the parasite would take into consideration the geo graphical range of the latter. Plants showing the particular symp toms would be obtained from different localities and the parasites isolated and compared. Thus a general idea would be obtained of the geographical distribution of the parasite and of the range of host plants which it could parasitize.
The complete elucidation of a plant disease involves much more, however, than the determination of the existence and habits of a parasite as outlined above. The presence of the parasite is only one of the conditions essential for the appearance of a specific parasitic disease. The more important factors and the manner in which they operate willfnow be indicated.
First as regards temperature. This factor determines in large measure the rate of growth and general vigour both of parasite and host. There is a certain temperature (minimum) below which an organism will not grow, a maximum above which it will not grow, and an optimum at which it grows best. These three temperatures constitute the so-called cardinal points for the particular organism. The cardinal points for the parasite are not in general the same as for the host plant, and the cardinal points for the disease itself are not the same as those of either host or parasite. Usually the range of temperature over which disease takes place is distinctly narrower than that over which either of the two organisms concerned will grow. The optimum temperature is well known in many cases to be different for host and parasite, and thus it is clear that the possibility exists of finding a temperature at which the host plant grows well but which is unfavourable for the development of the disease. The precise relationships are somewhat complex and have been fully worked out for a limited number of diseases only, but it is possible in general terms to classify diseases into the two main types, high temperature and low temperature diseases. An example of each will show the difference. The root-rot disease of tobacco is one of the low temperature type. Susceptible varieties of tobacco when grown in infected soil are badly diseased if the temperature rules low, whereas in the same soil the disease would not occur at a higher temperature. The wilt disease of cabbage
is of the high temperature type, and only occurs in seasons when the general ground temperature is high.
The water-relationships of the crop are of equal im portance with those of temperature and are to be studied in terms of the amount and distribution of rainfall, the water-retaining capacity of the soil, drainage, etc. Two factors which are of great importance in the initiation of parasitic attack are the relative humidity of the air surrounding the plant and the per centage of moisture present in the soil. The parasite requires the presence of a certain amount of moisture to start its growth processes. So it will often be possible for the plant pathologist to explain the occurrence of disease at one time or place and not at another in terms of these factors. The percentage of moisture in the soil affects not only the capacity of the parasite to grow but also the growth of the roots of the plant. It may therefore also influence the capacity of the roots to resist the attacks of the parasite.
is also a factor which influences the susceptibility of a plant to disease. The quantity of light received by the plant determines in large measure its vigour of growth, not merely as regards the amount of new tissue formed, but also as regards the mechanical strength and to a certain extent the chemical com position of the tissues of the plant. These qualitative differences influence the degree of resistance which the plant can offer to the attack of a parasite. For example the weak, pale, rather juicy, "forced" type of growth which results when plants are grown under conditions of feeble illumination—as when plants are grown in glasshouses during winter or when they shade each other by standing too closely together—predisposes them to attack by a variety of fungi ("damping-off" diseases). On the other hand, certain fungi (e.g., some of the rusts) attack their hosts with vigour only when the latter are growing under conditions of good illumination. Again, the symptoms of certain diseases (e.g., some mosaics) will only appear when the plant is exposed to light of a certain range of intensity.