CROP PROTECTIONS

THE CONCEPT OF DISEASE

A plant disease interferes with the normal structure and functions of plants and may affect the marketability or aesthetic value. Plant diseases can affect any part of the plant. They result from either an abiotic (non-living) agent or from a biotic (living) agent. Abiotic agents include environmental stresses that arise from temperature and moisture extremes, nutritional deficiencies and toxicities, and injuries caused by chemical or mechanical means. Biotic disease agents include the fungi, bacteria, viruses, and nematodes. The fungi cause the vast majority of biotic (infectious) diseases on all plants. Infectious plant diseases result from an interaction of a causal agent (pathogen) and a plant (host) when the time and environmental conditions are conducive for the interaction.

2.2: What is health?

It is the ability to carry out normal physiological functions at an acceptable level consistent to genetic potential. Normal physiological functions include:

  • Normal cell division, differentiation, and development,
  • Absorption of water and minerals from the soil and translocation;
  • Photosynthesis and translocation of photosynthates;
  • Utilization and storage of photosynthates;
  • Metabolism of metabolites and synthates;
  • Reproduction;
  • Storage of reserves for overwintering or reproduction.

2.3: What is Disease?

  • "It a malfunctioning process that is caused by continuous irritation. This process must result in some suffering, and produce symptoms"
  • "The term plant disease is properly applied to any deviation from normal growth or structure of plants that is sufficiently pronounced and permanent to produce visible symptoms or to impair quality and economic value."
  • Any disturbance of a plant that interferes with its normal growth and development, economic value, or aesthetic quality;
  • Any disturbance brought about by a pathogen or a consistent environmental factor which interferes with manufacture, translocation, or utilization of nutrients
  • Failure to reach full genetic potential due to the activities of another organism or environmental factor.

2.3.1: Disease is not a condition

A disease is ‘deeper’ than the symptom. A disease is the totality of the biological activity of all interactants both overt and covert. The term ‘pathodeme’ is used to express the altered metabolic result of the contributing interactants, but the pathodeme is not disease.

2.3.2: Disease is not the Pathogen

Pathogens are the causal agents of disease. Imprecise usage of terms has lead to the careless application. One hears " Phytophthora infestans is Late Blight of Potato". This mis-statement fails to recognize that the organism is not the disease and that disease cannot occur in the absence of a host.

2.3.3: Disease is not infectious

Following the above logic, because disease is the result of host and parasite interaction; only the parasite/pathogenic partner is infectious.

2.4.4: Disease is not mobile, is not disseminated

Propagules and inoculum are disseminated and the disease host may be transported; but it is incorrect to equate disease and inoculum when speaking of epidemiology or dissemination. Only inoculum is disseminated.

2.3.5: Disease and injury are not the same

Mowing a lawn may remove as much as 60% of the biomass of the grass and may cause wounding by the mower; but it is a single nonrecurring event that does not cause constant irritation. As such disease is not the result of tissue removal. However, one should not ignore the tremendous wound sites produced by tissue removal and their potential for entry sites for opportunistic parasites that may lead to disease.

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2.4: Loses caused by plant diseases

Over the past ten years, head scab on wheat (also called Fusarium head scab) caused by Fusarium graminearum has emerged as a significant pathogen on wheat and barley in Europe and North America. Infection of grains by this fungus can lead to significant levels of deoxynivalenol in the food supply. The Irish Potato Famine of 1845-1860 was due to the disease potato late blight, caused by the fungus Phytophthora infestans. Before the famine, there were eight million people in Ireland. In 1845-1860, a million Irish people died of starvation or disease, and an additional 1.5 million immigrated to other countries to avoid starvation. This disease also led to the birth of the science of Plant Pathology.

The last major famine due to Phytophthora infestans was in 1916, when 700,000 German civilians died of starvation because they were unable to protect their potato crops with the common copper fungicide used at that time. Potato late blight is still an important disease worldwide, and in the 1990s re-emerged as a major disease as a result of the migration of new strains out of Mexico.

A more recent epidemic that occurred in the USA in 1970 caused over $1 billion of losses (1970 dollars) in the corn crop. The disease was Southern corn leaf blight caused by the fungus Cochliobolus heterostrophus. The fungus moved very quickly through the major maize-growing areas of the US, causing losses of 80-100% of the crop in many fields. This disease made obvious the danger of genetic uniformity, as it became clear that this disease affected only corn plants that possessed Texas male-sterile cytoplasm (TMS), which was used in about 80% of the corn crop in 1970 to create hybrid varieties. It turned out that one race of the fungus, called Race T, produced a toxin that specifically attacked the mitochondria present in TMS cytoplasm, making TMS plants especially susceptible to infection by the Race T strain. Race T could complete a cycle of infection in a little over two days on TMS maize plants. The result was a very fast spread of the epidemic.

The examples so far have been of fungal caused diseases, but bacteria also cause many diseases. One that is relatively new to Europe is Erwinia amylovora, which causes the disease fire blight on apples and pears. It was accidently introduced to Europe about 40 years ago, first detected in England in 1957 and then in the Netherlands and Poland in 1966. Fire blight still is spreading in Europe, and appeared in Switzerland in 1996. In 1900-1910, 95% of pear trees in the San Joaquin Valley of California were destroyed by fire blight. Fire blight has the potential to devastate fruit trees and requires careful and intensive management. Antibiotics are applied in apple and pear orchards in North America to control fire blight.Examples of severe losses caused by infectious plant diseases is as indicated in the table below.

Table 1.2: Examples of Severe Losses Caused by Plant Diseases

Disease / Location / Comments
Fungal dieseases
1. Cereal rusts / Worldwide / Frequent severe epidemics; huge annual losses
2. Cereal smuts / Worldwide / Continuous losses on all grains
3. Ergot of rye and wheat / Worldwide / Poisonous to humans and animals
4. Late blight of potato / Cool, humid climates / Annual epidemics, e.g., Irish famine (1845-1846)

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