Biometeorology and Plant Disease

VOL. 44, No. 8, AUGUST 1963 499 Biometeorology and Plant Disease RICHARD D. SCHEIN Pennsylvania State University In mid-1962, publications of portions of Rachel in what intensity that threat is going to present Carson's Silent Spring in the New Yorker focused itself, and to know these things demands a more attention on the widespread usage of chemicals in accurate knowledge of complex interactions of modern agricultural practice. The insecticides re- parasite, host, and environment. ceived the largest portion of the indictment, but Thus, I would like to discuss that part which fungicides and other materials used in the control biometeorological investigations play in laying the of infectious plant diseases were not ignored. foundations for proper forecasting of plant disease Plant pathologists concerned with plant disease development. To avoid error in interpretation, I control have, for years, realized that they operate will work within the definition that biometeorol- in a twilight area between applied chemical con- ogy is a branch of ecology which studies the inter- centrations so high they damage plants, as well as relations between the chemical and physical fac- humans and animals, and so low they do not con- tors of the environment and living organisms. trol parasites. The federal and some state gov- Its bailiwick ranges from the bottom of the rhizo- ernments have regulations to guarantee residue sphere to the highest levels of the atmosphere levels probably not injurious to humans and ani- where pollens and spores may be found. Biome- mals at ingestion. These matters, and commen- teorology is further concerned with manmade en- tary on faults of Miss Carson's position and con- vironments. It is an interdisciplinary field involv- clusion, have been the subjects of book reviews, ing a wide range of atmospheric and biologic articles, and somewhat official rebuttals from the sciences. To me, it is autecology, redefined. agricultural chemical industry in its trade journals. This definition acknowledges that biometeorol- But no matter how the use of chemicals is ration- ogy is a branch of biology, that some important alized, there is considerable truth in the opinion biological responses are a function of the environ- that, in any given season on any given crop, more ment, and that ecological characterization of these chemicals are used than might actually be neces- responses constitutes biometeorology. We should sary. note, however, that those situations in which the Plant pathologists submit that without the living organism has effects on environment chemicals they could not maintain the quantity (which might be called 'feedback'), including its and quality of agricultural production required, own, are also of concern to the biometeorologist. any more than a physician could curb human disease without prescription formulae. But ex- 1. Some general comments on plant disease cessive use of chemicals is a fault in the art of plant disease therapy, for its practice is still largely Plant disease originates from many causes— empirical. Plant diseases are remarkably, inextri- weather, soil and nutrition factors, bacteria, fungi, cably, weather dependent. In the absence of quite viruses, animals, and genetics—but, regardless of precise knowledge—disease by disease—of this incitation, weather and the host's environment dependence, forecasting of occurrence is far from play large roles. My remarks will be mostly perfect. Today's regimens for the control of plant about fungous diseases, not only because great at- disease make certain the chemicals are present tention has been and will continue to be paid to when the disease threat presents itself. What we them, but because they make good subjects to il- want to know—need to know—is when, how, and lustrate various points. 1 Contribution No. 305 from the Department of Botany Plant disease development has a sequence, and and Plant Pathology, The Pennsylvania State University. that sequence is greatly conditionable. We first Presented at Symposium on Biometeorology, American recognize that host and parasite each has a genetic Association for the Advancement of Science, Phila- delphia, Dec. 27, 1962. system which determines the range, rapidity, and Unauthenticated | Downloaded 09/27/21 11:13 PM UTC 500 BULLETIN AMERICAN METEOROLOGICAL SOCIETY quality of its actions. Each of these systems is riod. Several biometeorological processes are subject to change through normal genetic mech- operative. A plant's predisposition to be dis- anisms. Within these genetic bounds, the game is eased after inoculation depends, in part, on inter- played, the organisms responding variously to the actions during Period I with nutrients; soil tem- physical environment, and, in the pathological perature, moisture, and reaction; atmospheric process, they respond to each other. The number temperature, moisture, and constituents; and light and degree of these interactions among host, para- quality, intensity, and perhaps photoperiod. site, and environment determine when disease oc- At point II the first inoculum arrives, for our curs, its quantity, its severity, and its epidemic fungus, as spores. The numbers (density of in- potential (or how fast it will advance). oculation), viability, genetics, and potential infec- Thus, Large's [1] equation for the progress of tivity of this inoculum is important. How some potato blight of these become part of biometeorological study will be partially covered under Period V. % = Ca(A-a) The arrival and deposition of inoculum at II is meaningless unless those spores have found sus- in which C is a constant, a is the proportion ceptible hosts and can germinate and their tubes of susceptible tissue that is infected, A is total penetrate the hosts and establish infections. susceptible tissue, and t is time, and which says Luckily (from man's point of view), this is one that the rate of disease development is propor- of the weakest links in the disease production tional to the product of the proportions of dis- chain; here is a place where environmental de- eased and healthy tissues, can be rewritten by pendencies are quite strong. Most fungous spores Van der Plank [2] as have an obligate free moisture requirement for rJr germination, germ tube growth, and penetration ^ = kx(l-x)f(T,H,S...) (Period III). These processes are also temperature dependent and so the interaction of time span in which x — a above; A becomes unity and and temperature of moisture on leaf surfaces be- f(T,H,S,...) is a function of temperature, humid- comes important. ity, host susceptibility, etc. The Period III situation may be further com- The environmental dependence of a generic fun- plicated by light effects, by the chance that a gous disease can be illustrated as in Fig. 1, the germ tube can 'find' an infection court, by the sequence of development in time. availability of such infection courts, and by the The line starts with seeding, or the beginning age and physiology of hosts. of host growth, and proceeds for a time during It is to protect against these events of Period which the seed germinates and/or growth occurs III that most fungicides are applied before the and susceptible tissue is produced. Sometime moisture period occurs, since the chemicals are during its life, pathogens, capable of inciting dis- prophylactic, acting as a barrier between the host ease, reach the plant's leaves (inoculation). This and parasite. Plant disease-protection regimens segment (I) we can call the pre-inoculation pe- and forecasting techniques are largely designed FIG. 1. The sequence of development of fungal plant disease. Unauthenticated | Downloaded 09/27/21 11:13 PM UTC VOL. 44, No. 8, AUGUST 1963 501 to make certain the plant is protected when the [6], and many others. The second reference con- disease threat presents itself. tains a bibliography of over 400 citations to work Period III is usually short, perhaps 2-48 hours on these matters. in length (depending on organisms and weather) The outlined process is obviously sequential; but it can reoccur as frequently as favorable one process is dependent on the success of previ- weather, once inoculum is available or begins to ous ones. Meteorological variables affecting the arrive. Successful penetrations and infections processes vary in their primacy; free moisture now enter an intimate relationship with the host may be most important at one stage, temperature (Period IV), and, logically, all of those factors at another, and wind at a third. But the con- playing roles in host development in Period I may clusion I want to draw seems obvious. The com- be operative. plexity of disease development derives from the We must add here the interactions of the ge- complexity of the interactions involved. Fore- netic-physiologic systems of host and parasite. casting, to be really accurate, must be based on These interactions determine the rate of coloniza- precise knowledge of a fantastically complicated tion of the host and the character of the interac- ecology. tion and thus, the time of appearance, character, and, perhaps, number, of symptoms. Here is 2. A word about epidemics where the disease is felt by the plant and by its producer. The word "epidemics" means in or on people, These same interactions and factors affect the so we are not quite correct when we talk of epi- time of occurrence and character of fungus fructi- demics of plant diseases, or of epidemiology—the fications : in our example, the production of spores. science comprising the study of epidemics. We And, the numbers of spores produced—per lesion, have a word in botany for this unarrested spread per field, per farm or larger area—have a marked of disease in plants—epiphytotic. Unfortunately, effect on subsequent cycles and the rate of devel- 'epiphytology' which should logically mean the opment of the epidemic. Epidemics do not occur study of establishment and spread of disease in unless there are several repetitions of the cycle.

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