Resistance is an inheritable characteristic that decreases the effects of parasitism. Its effectiveness depends on the combination of two factors:
- the level of expression of resistance;
- durability, or stability of resistance over time.

These two factors each have a particular determinism.

• Determinism of the level of resistance

Resistance and sensitivity are the two extremes of a host plant response set. Depending on the mechanism involved, the level of resistance will appear higher or lower, hence the distinction between absolute resistance and partial resistance.

Absolute resistance, also called "vertical", is due to a phenomenon of immunity or a mechanism of hypersensitivity. In the case of immunity, the plant is completely free from parasitism. This resistance may result from a lack of binding of the pathogen to the host or, in the case of viruses, from the absence in the host of an element or function essential for viral replication. Resistance to Mycovellosiella fulva , conferred by the “ gene Cf-2 ” , is a fine example of immunity; it is now overcome by race 2 present in many regions. The term "immunity" is often misused to denote resistance which manifests itself in the absence of visible symptoms, but which does not exclude the penetration of the pest.

When a hypersensitivity mechanism intervenes, the infection process remains localized and inactivated by the death of the infected tissues.

At high temperature, the mechanism is slower and symptoms may appear. Two examples are provided by the “ gene Tm-2² ” for resistance to Tomato mosaic virus (ToMV) and to Tobacco mosaic virus (TMV) and by the “ gene Mi ” conferring resistance to Meloidogyne spp.

Partial resistance, also called “horizontal”, is characterized by the decrease in the number of attachment points of pathogens on the host, the slowing of their growth and development in the tissues and the decrease in the number of infectious units. issued. Partial resistance to Phytophthora infestans , controlled by the “ gene Ph-2 ” , illustrates this situation. The overall result of these phenomena is a slower progression of the disease on the plant and of the epidemic in the crop.

Partial resistance supported by good cultural practices and reasoned phytosanitary protection can prevent the development of an epidemic.

The notion of partial resistance should not be confused with tolerance, which is an agronomic notion. The latter characterizes the behavior of a plant in which the parasite lives and reproduces, as in a susceptible plant showing typical symptoms of disease, but whose yield is not affected. However, it is common to speak of “tolerant varieties” in virology to characterize plants which allow active multiplication of a virus without showing symptoms typical of the disease, and whose yield is not affected.

• Determinism of resistance stability over time

The stability of high-level resistances can be extremely variable depending on the genes controlling them. It is only after many years of use that one can reliably assess the duration of resistance or the practical importance of adapting a pathogen to a given resistance.

The rapidity of appearance of new pathotypes is extremely great in certain pathogens such as Mycovellosiella fulva . In contrast, after many years of use in a wide variety of environmental conditions, some resistance has never been overcome, such as that to Stemphylium spp. On the other hand, there are many examples of resistance which, although overcome, continue to be of significant practical interest in certain cropping contexts.