Hemileia vastatrix

Coffee leaf rust. This disease is of interest because, when arabica coffee was taken as one pure lines to the New World, all of its pests and diseases were left behind in the Old World. This gave Latin America a commercial advantage and it now produces about 80% of the world's coffee. When the rust was accidentally introduced into Brazil in 1970, there were fears of a major disruption of the world supply. Fortunately these fears proved groundless, as the disease was easily controlled.

Leaf rust is also interesting in that its spores can be either wind-borne or water-borne. The former method of dissemination is clearly for the process of allo-infections from tree to tree. The latter is for Auto-infection from one leaf to another within one tree.

This disease is an apparent exception to the rule that vertical resistance will evolve only in the seasonal tissue of a discontinuous pathosystem, because coffee is an evergreen perennial. However, infection can only occur if there is free water on the leaf surface. During the tropical dry season, all infected leaves are shed, and the fungus dies with them.

Consequently, arabica coffee is functionally deciduous with respect to rusted leaves only. Hemiptera

An order of insects usually called 'bugs'. Many leaf bugs are important crop pests. The order also includes the bed bug, which is of interest in demonstrating the stability of natural pyrethrins. Hemp

See: Cannabis sativa. Hemp, Deccan

See: Hibiscus cannabis. Hemp, Manila

See: Musa textilis. Hemp, sunn

See: Crotalaria juncea. Herb

Any flowering plant that lacks woody tissues. Herbicide

Any chemical that kills weeds. Modern herbicides are often selective in the sense that they will kill some types of plant but not others. Typically, 2,4-D kills Dicotyledons and may be used safely on Monocotyledons crops such as cereals. Further selectivity has been obtained by the use genetically modified crops that are resistant to a specific herbicide. Herbicide injury

Traces of herbicide in sprayers or other equipment can cause injuries to crop plants. The symptoms can be very confusing and, if suspected, a specialist should be consulted. Herbivores

An animal that lives on plants, mostly grasses that can withstand grazing because their leaves grow from the base, and not the tip. The appearance of grasses, some 25 million years ago, led to an explosive evolution of herbivores. The evolution of humans, as hunter-gatherers, depended on the fact that the African savannah carries up to 20,000 kilograms of herbivores per square kilometre. At the other extreme, tropical rain forest carries only 5-10 kg/sq.km. It is no accident that rain forests have the fewest archaeological remains of hunter-gatherers, or that our hominid ancestors favoured open grasslands.

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