Epidemiological Competence

The two principle factors governing the level of parasitism in a matched host are the level of horizontal resistance in the host, and the level of epidemiological competence in the parasite. If the epidemiological competence is zero, there will be no parasitism, regardless of the level of resistance. If the epidemiological competence is maximal, the level of parasitism will be maximal if the resistance is minimal and the parasitism will be minimal if the resistance is maximal. These scales are...

Onsite screening

On-site screening means that the screening is conducted (i) in the area, (ii) the time of year, and (iii) according to the farming system, of future cultivation. The principle reason for on-site screening is that agro-ecosystems vary so widely. In particular, the epidemiological competence of most parasites varies from one agro-ecosystem to another. An agro-ecotype that is in perfect balance in one agro-ecosystem will be out of balance in another. This is because it will have too much...

Old clones

Clones that have been cultivated for centuries, even millennia, without crop protection chemicals, offer the best indication of the durability of horizontal resistance. The clones of the classic wine grapes have been cultivated for centuries without any crop protection chemicals. It must be appreciated that all the serious parasites of grapes are new-encounter parasites that were introduced relatively recently from the New World. These include phylloxera, Plasmopora, Uncinula, and Guignardia....

Potatoes in Kenya

Potatoes were introduced to the Highlands of Kenya in 1900 and, being free of blight, they were a very productive and popular crop for the next forty years. However, blight was accidentally introduced during World War II, and it proved devastating. Potato production all but ceased, particularly among subsistence farmers, who had neither the cash nor the expertise for fungicidal spraying. One old Dutch cultivar, known locally as Dutch Robijn Robijn rhymes with 'no pain' had sufficient horizontal...

Epidemiological isolation

It is worth noting that organic farmers benefit from an artificial epidemiological isolation. They are greatly protected by their neighbours' use of crop protection chemicals. If their neighbours were not spraying, say, their potatoes against blight and Colorado beetle, the organic farmers would probably lose their potato crops entirely because of the huge influx of parasites from those unsprayed crops. Their neighbours would also lose their crops . In this sense, many organic farmers depend...

Date palm

Seedlings of the date palm normally produce fruit that is fit only for feeding camels. Furthermore, this palm has a breeding cycle of six years, and its vegetative propagation is very slow. The palm is also dioecious, meaning that each palm is either male or female, but not both. Choosing a male parent on the basis of fruit quality is possible but very difficult. Date palms also occupy a lot of space. Amateurs could be involved in the screening of existing populations of seedling date palms...

Reversible and Irreversible Ecosystem Damage

We must make a clear distinction between reversible and irreversible damage to ecosystems. If the damage is within the resilience capabilities of the ecosystem, it is reversible, and the ecosystem will recover. Occasionally, however, an ecosystem can suffer damage beyond the limits of its resilience, and beyond its capacity for recovery. Some ecosystems are fragile, and they can easily be triggered into collapse. During the Holocene pluvial period 12,000-9,000 years ago , Lake Chad was the size...

Pathotypes and Pathodemes

Robinson 1969 proposed a system of naming the strains of both hosts and parasites that are defined by criteria of parasitism. This system was intended to replace the antiquated plant pathological terminology which used the terms 'physiologic race' and 'pathologic race' to describe pathologically defined strains of pathogens. There was no corresponding terminology for the host. Entomologists spoke of 'biotypes' when describing insect strains defined by resistance in the host. They too had no...

Potatoes in Europe

The history of blight Phytophthora infestans of potato Solanum tuberosum in Europe falls conveniently into periods of approximately forty years, starting in 1845. This history reveals a remarkable fluctuation in the levels of horizontal resistance. Before 1845 The edible potato evolved in South America, and the blight fungus evolved in Mexico. When the two were brought together, first in New York, and then in Europe, this was a new-encounter disease, which revealed a serious crop vulnerability...

Demonstration of a Genefor Gene Relationship

Person Habgood Differential Interaction

A gene-for-gene relationship can be demonstrated genotypically or phenotypically. A genotypic demonstration of a gene-for-gene relationship requires genetic studies in both the host and the parasite. Such studies can be a lengthy business, often requiring many years of painstaking work. Genetic studies in the parasite are often extremely difficult, as with aphids or rusts, for example. In many other parasites, genetic studies are impossible because the reproduction is entirely asexual. A...

The vertifolia effect

The fourth disadvantage to breeding for vertical resistance has already been mentioned. It is insidious, and largely unappreciated, but dangerous for this very reason. This is the decline in the level of horizontal resistance that slowly but inexorably occurs during breeding for vertical resistance. Vanderplank 1963 first recognised this phenomenon, and he called it the 'vertifolia effect' after a potato variety of this name. It was only after its vertical resistance had broken down that it was...

The Person Habgood differential interaction

The Person Differential Interaction

Having applied the Habgood 1970 nomenclature for the host and parasite differentials to the pairs of genes in a gene-for-gene relationship, Robinson 1976 rearranged the Person 1959 differential interaction on the basis of the Habgood nomenclature. There was then a greatly increased simplicity, and he called this the Person Habgood differential interaction Fig. 4.4 . Intriguingly, the patterns within the differential interaction are reproducible with the techniques of cellular automata see 2.2...