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Green-seeds.com:
publications & research: articles
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DISEASE RESISTANCE IN TOMATO: PRESENT SITUATION AND HOPES
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H. Laterrot
INRA - Avignon- France
 All types of pathogens
can be found in tomato crops. Genetic resistance can be used to control
an ever increasing number of pathogens. Most resistance traits actually
used are monogenic and dominant, and breeders are generating F1 hybrids
accumulating several genes. Combining several genes in open pollinated
varieties requires more time and breeders need to preserve the exclusivity
of their varieties. In most cases interesting resistance genes are
linked with defects that are masked in the F1 hybrids.
Wild species of
Lycopersicon were exploited since the 1940s by breeders as sources
of disease resistance. Most modern varieties result from interspecific
crosses. The importance of wild species allied to the tomato is always
increasing, not only as sources of resistance to pathogens, but also
for environmental adaptation and fruit quality.
Present situation
In practice, 13
different pathogens are now controlled by genetic resistance. Their
level of expression and their stability are widely variable. The interest
in these resitance traits depends of the cultural conditions.
In greenhouses
conditions of temperate regions, the cultivated F1 hybrids generally
are resistant to Verticillium dahliae (gene Ve), Fusarium
oxysporum f.sp. lycopersici, 2 pathotypes (genes I and
I-2), Tomato mosaic virus (= TMV) (gene Tm-22 )and
many time to Fusarium ox. f.sp. radicis lycopersici
(= FORL) (gene Fr1), Fulvia fulvia (= Cladosporium) (series
of genes Cf) and Meloidogyne spp (= rootknot nematode) (gene
Mi).
Under field cultivation
in temperate areas open pollinated varieties are cultivated as are
F1 hybrids with a reduced number of resistance genes (Ve, I, I-2).
In particular conditions some varieties possess the resistance gene
Mi and sometimes resistance to Pseudomonas syringae pv.tomato
(gene Pto).
For the subtropical
and tropical conditions resistance to other diseases such as Stemphylium
spp (gene Sm), Ralstonia solanacearum (= Pseudomonas
solanacearum) (genetic complex) is used.
Partial resistance
introduced in the tomato is rarely used, for example resistance to
Phytophthora infestans (gene Ph-2) and Pyrenochaeta
lycopersici (gene pyl). These resistances have a partial
action and are difficult to select. This situation shows the importance
of developing reliable methods to test the plant material. For this
the use of molecular markers is becoming useful.
Hopes for the next years 
The wild species
allied to the cultivated tomato are the sources of new hope to control
other pathogens.
Some varieties
are arriving with very interesting resistance. The breeders are currently
working to introduce in their improved material, resistance to :
- Leveillula taurica (= Oïdiopsis taurica) controled
by the gene Lv discovered in Bulgaria
- Oïdium lycopersicum, resistance from different sources
shown in the Netherlands
- Alternaria dauci f.sp.solani, the causal agent of
early blight. The partial resistance selected in North Carolina seems
the most interesting
- Clavibacter michiganensis (= Corynebacterium), partial
resistance from different sources accumulated in France, in some canning
tomato lines
- Xanthomonas campestris pv. vesicatoria, partial resistance
discovered in Florida and Bulgaria
- Tomato spotted wilt virus (= TSWV), controled by the gene Sw-5
introduced in South Africa in very interesting material
- Tomato yellow leaf curl virus and other geminiviruses, resistance
from different wild species. The most studied being the high level
resistance from Lycopersicon chilense demonstrated in Israel.
Many breeders are using the resistant populations constituted in France
with a large international network supported by the European Community
This list is not
exhaustive, and breeders are attempting to use other resistance genes
from wild Lycopersicon species. Genetic transformation is also
being used with different approaches, more specially to obtain Cucumber
mosaïc virus resistant material.
The disease resistances
are not the only one objective for breeders. A few years ago, the
possibility to improve fruit shelf life was demonstrated. It is now
necessary to accumulate the maximum number of resistance genes with
this commercial quality and also to work to improve flavor and nutritive
quality. All progress being realized in F1 hybrid combinations the
growers have to buy the seeds every year and high price of these obliges
the improvemant of plantlet production. This is the progress necessary
to realize quickly in developing countries to decrease the high seed
number necessary to obtain one plant.
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