The development of plant disease epidemiology can be attributed to the disease outbreaks that occurred in the past and have changed the course of history of man. The defeat of Peter the Great, the discovery of the Bordeaux mixture, the conversion of the British from a nation of coffee to tea drinkers, the destruction of chestnut trees in the Americas, and the great loss in the corn industry of the USA in the past were due to several disease outbreaks caused by different plant pathogenic organisms.

The defeat of Peter the great of Russia in his fight to gain control of certain warm-water ports on the Black Sea in 1972 was due to the ergotism. Ergotism was due to the ergot of rye caused by Claviceps purpurea. This fungus invading the grain produces an ergot that contains many alkaloids including a hallucinogenic drug, LSD. When grains with ergot are ground and baked to bread, the problem begins.

Small amounts of alkaloids can induce abortion in cattle and in man. Larger amounts can cause can fingers and toes to tingle, and high fever that can result to derangement and even death if the fever persists. The patient may also experience hallucinations and even gangrene of the extremities after ingesting contaminated rye bread.

Sacer ignis-the holy fire was also an epidemic of ergotism during the 857 AD in the Rhine Valley in Europe killing thousands of people. The monks of St. Anthony were able to relieve the symptoms of the disease in 1039 AD in France and the disease was known as St. Anthony’s fire. The monks fed the patients with ergot-free bread together with spiritual administrations. An epidemic of ergot of rye has also lead to the accusations of witchcraft in Salem Village, Massachusetts and in Fairfield Country, Connecticut in 1962 as shown by some evidences. In 1951, weather conditions have allowed ergot to develop in rye in France. By fall, 200 cases of severe illness, insanity and four cases of deaths have been reported in Pont-St.Esprit.

Bordeaux mixture-a mixture of copper sulphate and lime was proven to effectively control potato late blight that caused the Irish famine in the early 1840’s. The potato late blight disease is caused by Phythopthora infestans favoured by cool, wet weather. The full effects of the disease was felt by 1846, thus the Irish famine.

The disease, however, was not limited to Ireland. There were reports of the occurrence of the disease in the northern United States and northern Europe. By 1855, the Irish population dropped by 3 million-1 million dead due to starvation and associated maladies, and 2 million as emigrants to the United States, Canada, and other countries.

In 1882, Bordeaux mixture was discovered in France during the World War I. But due to the war needs, military leaders did not release the copper since potato and grains were supplied for the army in 1916 and 1917. Military collapse of Germany in 1918 was due to the decline of morale of the army because although they were not hungry but their family were starving.

The British in Ceylon, now Sri Lanka, who were coffee drinkers, became a nation of tea drinkers when Hemileia vastatrix, the causal agent of the devastating coffee rust came to existence.

By 1870, 200,000 ha of coffee were planted by the British in Ceylon with 50 million kg of coffee beans for export per year. A year ago, Reverend M.J. Berkeley was able to describe and name Hemileia vastatrix, the disease results to the premature falling of leaves in about a hectare of land and suggested that sulfur be immediately applied to prevent further spread of the fungi. No one took heed on Berkeley’s suggestions and by 1874; the disease has become widespread on the island and has reduced 55% of the total yield four years after. In 1880, Henry Marshall Ward arrived in Ceylon and studied the fungi, Hemileia vastatrix, and showed an effective way of controlling the disease but due to the high costs of the control method, farmers started to plant tea bushes. Tea became popular in Britain with the demise of coffee in Ceylon. Coffee rust also destroyed coffee plantations all over Southeast Asia and India and destroyed the whole continent’s coffee industry in ten years.

The destruction of chestnut trees in the Americas was caused by Cryphonectria parasitica, formerly known as Endothia parasitica responsible for the chestnut blight epidemic that has caused a major effect in their industries. Chestnut was a major forest species in the United States contributing 25% of the 100 commercial hardwood species in the southern Appalachian region. The nuts were good sources of food for humans and wild life while the wood was used in furniture, home and fences, as firewood, as decay-resistant poles, and as railroad tiles. The highly successful leather tanning industry was also a product of chestnut since chestnut was the major source tannin during that time.

In 1904, H.W. Merkel observed that the chestnut trees in the Bronx Zoological Park in New York City were dying caused by an exotic fungus, then Endothia parasitica. The fungus was brought to the United States and by 1911, the blight had spread over New Jersey and parts of New York, Connecticut, Massachusetts, Rhode Island, Delaware, Virginia and West Virginia and the pathogen continued to spread. As a result, entire communities of the Appalachians turned to other enterprises since their major source of tannin has disappeared. Thirty billion board ft was estimated to be the loss of their lumber.

Epidemic leaf blight hit throughout the corn-producing areas of the eastern United States in the summer of 1970. February of the same year, the southern corn leaf blight, caused by Bipolaris maydis, was found in hybrids that have exhibited previous resistance. This was alarming since 85% of the total corn planted acreage the US were planted with this hybrid.

Hypersusceptibility to Bipolaris maydis by the hybrids was attributed to the use of the Texas cytoplasmic male sterility technique in producing the hybrids. The hypersusceptibility of Tcms hybrid was first discovered in the Philippines by Mercado and Lantican in 1961. In 1969, Race T, a new race, of B. Maydis invaded the Corn Belt. Race T was highly virulent on corn with Tcms but mild on corns with normal cytoplasm.

By May 1970, Southern Corn Leaf Blight has invaded the southern United States and was moving northward due to weather conditions by June. Since 85% of the corn planted was susceptible, losses ranged from 10-30% resulting to 15% of the U.S. corn crop or about 20 million metric tons of corn (about $1 billion) was lost.

These five plant diseases are examples only of epidemics caused by plant pathogens. Understanding of the multitude of biological and environmental factors and the historical and sociological lessons that can be derived from these epidemics should help us better in managing plant diseases in the field.

Plant disease epidemiology was not recognized as a discipline of plant pathology until the 1960’s. The appearance of J.E. Vanderplank’s Plant Diseases: Epidemics and Control served as the catalyst for the development of plant disease epidemiology. The book provided the first comprehensive treatment of the description and quantification of plant disease epidemics.

Phytopathology became a discipline only in the late 1800’s and trends that would lead to epidemiology were not evident until the twentieth century.

Duhamel’s work in the eighteenth century discussed the epidemiology of the Death, a disease of saffron crocus, caused by now Rhizoctonia violacea and causes epidemics on plants and provided control recommendations for the disease. This was the first known published work of plant disease epidemiology but received little attention in the scientific world.

In 1850’s, the dawn of the science of plant pathology, the concept of an epidemic was exemplified in the writings of Julius Kühn. In 1901, H. Marshall Ward’s Diseases in Plants displays high perception in epidemics.

It was Ernst Gäumann’s Pflanliche infektionslehre (Principles of Plant Infection), the first comprehensive work in plant pathology concentrating in botanical epidemiology that emphasized the uniqueness of each plant disease epidemic. He also discussed the concept of “infection chain” providing the foundation for the analysis and understanding the components of an epidemic.

With Gäumann’s book together with the development of modern computers, and the growing awareness of plant pathologists in mathematics, statistics, and ecology in the 1950’s, the epidemiology of plant disease as a quantitative discipline took shape. The “birth” of modern botanical epidemiology occurred in 1960 when J.E. Vanderplank, in the chapter “Analysis of Epidemics” of his Plant Pathology, Vol. 3. Edited by J.G. Horshall and A.E. Dimond, took the quantitative approach and the logistic equation was first systematically applied in plant disease epidemics.

In 1963, a NATO Advance Study Institute in Pau, France, “Epidemiology of Plant Diseases” was held. Several international conferences, symposia and workshops soon followed over the years and were hosted by different organizations all over the world. Since then, an increasing number of scientists have made significant contributions to plant disease epidemiology.

Plant disease epidemiology is now a vital and growing discipline that will continue to play a role in understanding plant diseases and providing strategies for successful management of plant diseases.

In every field, weeds are always present. But what exactly are weeds? Well, some people define weeds as an unwanted, undesirable and useless plant. But no plant is completely useless. We may then define weeds as plants that are unwanted at a particular time and place and whose economic use has not yet been discovered. A plant species can only be called as weed if humans have not yet found a use for it. “One man’s crop maybe another’s weed.”

Some plant species though occur 99% as weeds in some fields. Echinochloa spp. and Monochoria vaginalis in rice fields for example. R. cochinchinensis and Cyperus rotundus in corn and vegetable fields respectively are considered as weeds. 10% of the 300,000 angiosperm species behave as weeds 99% of the time.

The unique characteristics of weeds have made them difficult to control in the field. They have excellent adaptations to disturbed environment and occupy the ecological spaces left open in agroecosystems. Other characteristics of weeds include their rapid vegetative growth, reproduce rapidly and mature early, very prolific and produce plenty of seeds, can survive and adapt to adverse conditions, dormancy of propagules or can be induced to dormancy under favorable environments, and are adapted to crop competition.

Rapid Vegetative Growth and Reproduce Rapidly and Mature Early

Weeds have numerous tillers for grasses, rapid tuber and shoot formation for sedges, and faster stem elongation and branching for broad leaves. Also, weeds are able to reproduce sexual and asexually. These have allowed the weeds to be able to maintain high population densities if not managed effectively. They also mature early so they are able to reach their reproductive period at a lesser time, hence more plants capable of to reproducing.

Very Prolific and Produce Plenty of Seeds

R. cochinchinensis can produce more than 700 tillers and branches and can produce inflorescence. Perennial weeds can reproduce rapidly through vegetative means through tubers, rhizomes, and stolon. Scirpus maritimus, a perennial sedge, can produce more than 100 dormant and non-dormant tubers in one cropping season in irrigated rice paddies.

Ability to Survive and Adapt to Adverse Conditions

Weeds are capable of resisting drought and excessive moisture stress. Large crabgrass (Digitaria sanguinalis) form contractile roots and arrests its growth during extremely dry conditions and resumes their normal conditions until a favorable condition is met. The common purslane (Portulaca oleracea) incline their leaflets upward to reduce exposure to sun during dry conditions thus reducing excessive moisture loss due to transpiration.

Propagules Possess Dormancy or Can be Induced to Dormancy

Dormancy is a mechanism that enables the species to survive under unfavorable conditions. This mechanism is common to weed species and until a favorable condition for growth is observed.

Adapted to Crop Competition

Weeds have proper synchronized germination. They are able to germinate at the right time in favorable environments. Their seedlings are fast growing and can be rapidly established. Their quick response to moisture and nutrient availability make them well adapted to crop competition in the agroecosystem.

It is very important to know the characteristics of weeds so that proper and effective weed management measures can be design to solve weed issues. What even makes it difficult is that weeds are also plants, like the crop planted in the field, anything that can harm them can possibly harm the crop as well. The soil too is a seed bank of thousands of weeds so it is expected that a weed can grow in empty spaces in the agroecosystem. Weeds are also are important in disease development because some weed species are alternate hosts of some pathogens. Weeds also provide a niche for other insect pests in the field, hence, controlling them is a very important management practice in the field.