Genetic answers to plant health problems
Wild plants show high levels of resistance to most pests and diseases.
In the search for high yields and extremes of flower shape and colour,
plant breeders have often failed to include this wild plant resistance
However, in crops such as antirrhinum, lettuce and tomatoes, one or
more resistance genes have been deliberately incorporated to give
protection against rust, downy mildew and tomato mosaic virus
respectively. However, the disease organisms competing against the
resistance may overcome the genetic barriers and the crop thus again
becomes infected. Growers may sow a sequence of cultivars (such as in
lettuce), each with different resistance genes, in order that the disease
organism (such as downy mildew) is constantly exposed to a new
resistance barrier, and thus limit the disease.
|Table 16.3 Some examples of resistance to pests and
in vegetable crops
Examination of the genetics of wild plant resistance usually shows that
there are several (often many) genes contributing to the overall resistance
effect. The complex nature of the resistance prevents the frequent development of new strains of diseases that could seriously affect the
plant. Gardeners and professional horticulturist alike are increasingly
looking to choose cultivars with proven long-term resistance as a feature
that is as important to them as yield and plant quality, etc.
Recently introduced cultivars of cabbage such as ‘Kalaxy’ are claimed
to have stable resistance to club root, while cultivars of potatoes such
as ‘Sarpo Axona’ have until now shown very good leaf resistance and
very good tuber resistance to potato blight. Looking at plant resistance
with a slightly wider view, it should be noted that some strains of fungus
are specific to particular families of plant. For example, a Fusarium
wilt strain attacking tomatoes (member of the Solanaceae
family) will not carry over to a subsequently planted crop of pinks and
carnations (members of the Caryophyllaceae).
Vegetatively propagated species, such as potatoes, and tree crops, such
as apple, which remain genetically unaltered for many years are now
being bred with high levels of ‘wild plant’ resistance (to blight and
powdery mildew respectively on potato and apple) so that the crops may
resist these serious problems more permanently.
Crop resistance to insects is now being more seriously considered by
plant breeders. Some lettuce cultivars are resistant to lettuce root aphid
s). A few carrot cultivars have some resistance to
carrot fly (Psila rosae
). A new apple cultivar has shown resistance to
apple aphids. Table 16.3 illustrates some vegetable cultivars showing
their resistance to various diseases and pests.