Leek and shallot 375
the pure stand by 10%, probably as a result of an optimized exploitation of the
resources. The quality of the leek, however, was reduced.
Leek rust (Puccinia allii) is now also difficult to control. Experiments in which no
insecticides or fungicides were applied were carried out to assess the effects on thrips
populations and infection by leek rust when leek crops were undersown with
subterranean clover (Trifolium subterraneum). To evaluate the economic aspects of
this approach, both the quality and quantity of the leeks produced in the two systems
were compared. Undersowing leeks with clover drastically reduced thrips infestations,
which was reflected in improved quality of leeks at harvest. Leek rust incidence was
also reduced slightly by undersowing with clover and the quality of the leeks at
harvest was also better. Although the quality of the leeks was improved when the
crop was undersown with clover, the quantity of crop produced was reduced considerably
as a result of plant competition.
Legutowska and Tomczyk (1999) studied differences in the development of thrips
on leek monocropped and leek intercropped with white clover. Intercropping reduced
the number of thrips. Chemical analysis of leek sampled from both crops were
conducted to estimate the contents of some nutritional substances (carbohydrates,
vitamin C (ascorbic acid) and nitrogen), and content of dry matter was also estimated.
The chemical analyses were performed separately for the white part of the leek and
for the green part (leaves). Leek intercropped with clover was richer in vitamin C
than monocropped leek. Analysis of monocropped leek indicated rather higher reducing
sugars and more soluble sugars. Analysis of initial colonization by Thrips tabaci
adults in leek interplanted with clover indicated that colonization rates in the intercropped
leek plants were lower in comparison with the leek monocrop. Seventy per cent of
the newly established thrips adults were found on the monocrop leek plants. After
cutting the clover around leek plants, the thrips suppression persisted. This supports
the conclusion that attractiveness or nutritional quality of the leek plant for Thrips
tabaci is reduced as a direct result of the interaction of leek with clover (Belder Eden
and Elderson, 1998).
22.3.14 Protected cultivation
The large number of leek cultivars now available make it possible to sow the crop in
Western Europe from December to June. Harvesting takes place from June to May in
the next year. The cultivation of leek in Europe is divided, according to the time of
harvest, into three main periods, i.e., summer, autumn and winter. The ‘winter’ leek
can be harvested until early April and kept in cold stores for a few weeks. Profitable
yields of early leeks grown under protected cultivation (June/July) were increased by
double feeding with Nitrophoska Permanent (15:5:20:2) combined with irrigation
and temperatures of 18–20 ∞C compared with 12–14 ∞C or 14–16 ∞C. Earlier crops,
by 2–3 weeks, could be obtained by forcing under perforated plastic (500–700 holes/
m^2 ) removed at the end of May before earthing up (Will, 1979).
22.3.15 Diseases
White-tip
White-tip disease, the most important leek disease in Europe during the winter, is
caused by Phytophthora porri Foister. Infected leaves show papery-white local lesions,
sometimes surrounded by dark-green watersoaked zones. Sporangia can develop in