Leek

Small lesions form on leaves and stems and enlarge into tan-to-purple blotches that collapse the leaf, cutting marketable shank quality and green-top appearance. The pathogen overwinters in allium crop residue and becomes damaging when temperatures are warm (roughly 77-85F) with high humidity and long leaf-wetness periods.

The same oomycete that attacks onion and garlic infects leek foliage, and its development is promoted by cool, moist conditions that are common through much of a long leek season. Severe outbreaks blight leaves, stunt shank development and predispose tissue to secondary rots.

White rot is most severe on overwintered leeks, because the cool, moist soil conditions that favor leek growth (infection between roughly 50-75F, optimum 60-65F) are also ideal for the pathogen. Sclerotia can survive in soil for over 20 years even with no host present, so a single introduction can remove a field from allium production for a generation.

Rust produces reddish to dull-orange oval pustules on leaf blades that coalesce until severely affected leaves yellow, dry out and die, reducing shank size and green-top marketability. The pathogen is active in cool weather between about 57 and 75F, with optimal infection near 59F and 100% relative humidity for at least four hours.

This invasive fly, which has spread rapidly into new production areas since the mid-2010s, damages leek more than most other allium crops. Females puncture leaves in distinctive rows of white dots and larvae mine downward into the shank and leaf sheaths where they pupate, rendering the harvested product unmarketable.

Thrips pierce plant tissue and remove liquids, with immatures feeding in the protected space between leaf sheaths and the stem and adults feeding on mature tissue, producing whitish or chlorotic blotches. Extended feeding reduces plant size and increases leaf and shank rots, and pressure escalates under hot, dry conditions.

Larvae of this invasive moth feed on the tops of young leaves and inside folded leaves, leaving a series of holes and frass as they move toward the plant centre, and occasionally mining to the base. Injury rarely kills plants but sharply reduces marketability, so even minor damage causes real economic loss, with two to three generations building damage across the season.

Overwintering hardy leek varieties for late-winter and early-spring marketing exposes a mature standing crop to freeze-thaw cycles, ice encasement and frozen ground that blocks lifting. Half-mature plants of hardy varieties stand winter freezing only with protection such as straw or salt-hay mulch in colder climates, and unprotected stands can be lost outright.

Leek is a slow crop that needs greenhouse seeding roughly 2.5 to 4 months before field setting and then grows slowly for the first two or three months in the field before shanks bulk up. In short-season or unpredictable climates the crop can fail to reach the 1-inch minimum base diameter before hard freezes, leaving undersized, low-value shanks.

Leeks typically hold quality only 2 to 3 weeks in storage even at optimum conditions near 32F and 95-100% relative humidity, so there is little ability to hold product waiting for a better price. Every unit also carries heavy handling cost, since soil clinging in the leaf sheaths must be power-washed off and product is trimmed and bunched by hand before packing.