Measurement of Faba Bean Diseases
There are three major parameters for the measurement of faba bean diseases; disease incidence (number of infected plants), disease severity (area of infected tissue), and yield loss (decrease in crop yield due to disease). In addition, the characteristics of disease lesions (infection types) which reflect the interaction between genes for resistance in the host and genes for virulence in the pathogen are used to evaluate different host materials for their disease reaction. Different infection types are normally associated with various degrees of chlorosis (yellowish haloes around lesions, Fig. 16), necrosis (dead tissue), and sporulation (Fig. 17).
Fig. 16. Presence (left), and absence (right) of yellowish haloes around necrotic alternaria spot lesions on two faba bean lines.
Fig. 17. Sporulation of Botrytis faba on a susceptible faba bean leaf in the field.

Using these parameters and characteristics, a faba bean line is considered resistant to a disease (Fig. 18) if plants in that line are able to discourage spore germination, penetration, growth, and sporulation of the pathogen. On resistant genotypes, lesions are flecky and small and their development is sometimes hindered by a hypersensitive reaction indicated by necrotic dead tissues surrounding the lesion (Figs 19, 23). This reaction efficiently cuts off nutrients and so restricts fungal growth. Although the actual damage to the host is slight, varieties that have been released on the basis of this type of resistance only, have often been shown to carry major genes with a highly race-specific interaction with the pathogen. Therefore, it does not seem reasonable to base disease measurement on infection type only as resistance-breaking races have frequently been associated with hypersensitivity.

Many systems have been developed for disease measurement. The following scales, which are based on disease incidence, severity, yield loss, infection type, and sporulation of the pathogen are suggested for disease measurement.

Fig. 18. Chocolate spot-susceptible (left) resistant (right) faba bean lines.

Fig. 19. From left to right, ascochyta blight-resistant, moderately resistant, and susceptible reactions on stems of three faba bean lines (Note hypersensitive necrotic reaction on left).

Fig. 20. Ascochyta blight-resistant (above), and susceptible reactions (below) on faba bean leaves. Note hypersensitive reaction restricting lesion spread on the above.
In some instances, individual plants within a host population may either be resistant or susceptible (Fig. 20), whereas in some other cases, a spectrum of reaction ranging from highly resistant to highly susceptible is observed (Fig. 21). These two possibilities may be due to segregating plant populations, seed mixtures, or outcrossing. However, in some other case a range of disease reaction may be observed on individual plants. this may result from the presence of different races or pathogenic variablities in the population of the pathogen ( Figs 22, 23 ).
Fig. 21. Highly resistant (left), moderately resistant (center), and highly susceptible chocolate spot reactions on faba bean leaves.

Fig. 22. A spectrum ranging from low (top) to high chocolate spot reaction (bottom), induced by two races of Botrytis fabae on five faba bean lines.

Fig. 23. Mild (left), and severe (right) hypersensitive reactions which might be induced by two different isolates of Botrytis fabae on the stem of the same plant. (The severe hypersensitive reaction on the right suggests a highly race specific interaction, and that lines of this nature may not carry genes for durable resistance).
 
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