376 Handbook of herbs and spices
wet lesions and may release 10–30 zoospores, while zoospores are formed when the
leaves dry up and may survive for a long period. Harvest losses may be severe; in
some cases, total crop loss is reported. De clereq and Bockstaele (2002) reported that
a two-step chemical control method can be used. During September and October
maneb is used preventively; when the first symptoms are visible, more systemic
products such as benalaxyl or metalaxyl are used curatively.
White rot (Sclerotium caplvorum)
Randy Baker (1991) reported that this soil-borne fungal disease can be devastating if
present in farm soils. The fungus survives as sclerotia in the soil for long periods.
Leeks should be grown on land that has not grown an onion family crop recently.
Sanitation through cleaning of field equipment and disposing of cull leeks away from
production areas is important in preventing the spread of this disease. The first signs
are yellowing and dying back of the leaves beginning at the tips and progressing
downwards. Young plants wilt and collapse and are easily dislodged from the soil,
revealing a dense white mass of mycelium in which minute black sclerotia are embedded.
Cool, wet growing seasons favour the development of white rot.
Leek rust
Leek rust (Puccinia porri G. Wint., syn. P. allii F. Rudolphi) causes severe damage
on European leeks. As the crop is now cultivated all year round, the uredo stage of
leek rust is present throughout the year. During winter, low temperatures inhibit the
formation of uredosori (the bodies that produce urdeospores, one of a possible five
types of rust spores). As soon as the temperature increases in spring, the epidemic of
leek rust starts again. The disease develops most frequently under conditions of high
humidity and low rainfall, while immersion of the spores in water reduces their
viability. The highest infection efficiency occurs at 100% relative humidity (RH) at
10–15 ∞C and temperatures above 24 ∞C and below 10 ∞C inhibit infection. The
economic threshold for leek rust is low, as all leaves are prone to damage and leaf
removal is not practical. A regular spray schedule with protectant fungicides (e.g.
maneb or zineb) should give adequate protection (Schwartz and Mohan, 1995). Spraying
fenpropimorph, either alone or in mixtures with maneb, provides a good control.
Compounds of the triazole group – tebuconazole and epoxiconazole – are also effective;
treatments with propiconazole resulted in outstanding control.
22.3.16 Insect Pests
Leek moth
Kristen Callow (2003) reported that the leek moth (or onion leaf miner), Acrolepiopsis
assectella Zeller (Lepidoptera: Acrolepidae), a pest of Allium native to Europe, was
first positively identified in Eastern Ontario in 1993. The distribution of the pest
includes Asia, Africa, Europe and Canada. The leek moth is considered a serious pest
in some parts of Europe, with levels of infestation up to 40% in areas where the insect
has several generations per year. Where generations are limited to 1–2 per year, the
pest is sporadic and causes little economic damage. Surveys conducted in 2001 by
the Canadian Food Inspection Agency (CFIA) indicated that the insect is present and
established in a localized area in Eastern Ontario and Western Quebec. Leek is the
preferred host of the pest, though other Allium crops can be attacked. The larvae will