Articles
BREEDING ALLIACEOUS CROPS FOR PEST RESISTANCE
Needless to say that I share these views, that I look forward for the release of resistant cultivars, and that I hope for a similar success with the improvement of short-day onions as well as other alliaceous crops.
Today, I shall review the state of the art with regards to breeding alliaceous crops for resistance to pests, provide information on sources for resistance within the genus Allium and list references for screening techniques for resistance tolerance, which are readily available for use in designated breeding programs.
Much like other plant species, alliaceous crops are prone to attack by a large number of pests.
These include insects, nematodes, mites, fungi, bacteria, viruses and mycoplasmas.
In recent reviews, Walkey (1990) described over 20 different viruses isolated from the four major crops: onion, garlic, leek and shallot [Table 1]. Entwistle (1990) listed over 10 root diseases of allium [Table 2], Maude (1990 a, b) characterized bulb and foliage diseases [Table 3], and Soni and Ellis (1990) listed over 100 species of insect pests of onions and other alliaceous crops [Table 4]. The above information clearly shows a tight pest-plant species relationship, which is demonstrated by the following two examples: Resistance to onion fly is quite common in A. fistulosum but very rare in A. cepa (Van der Meer, 1978) and tolerance to Yellow Stripe Virus is common in A. ampeloprasum var. kurrat but not in A. ampeloprasum var. porrum (Van der Meer, 1984). In addition, plant-pest relationships are strongly affected by both the specific genotype within each species, as demonstrated by the list of resistant/tolerant onion cultivars [Table 5], and the environment.
Certain pests have their most marked impact in warm climates, e.g., nematodes (Meloidogyne spp. and Ditylenchus dipsact), pink root (Pyrenochaeta terrestris) and purple blotch (Alternaria porri). Some thrive under cooler conditions, e.g., smut (Puccinia porri), and leaf blight (Botrytis squamosa), while others infect alliaceous crops in a variety of climates, e.g., white rot (Sclerotium cepivorum), downy mildew (Peronospora destructor), and thrips (Thrips tabaci) (For a recent review see: Rabinowitch & Brewster, 1990).
Despite impressive achievements in the agrochemical industry, pests still have the upper hand in the battle for food.
Hence, the use of partially or completely resistant cultivars to reduce pest damage is an attractive proposition.
It would increase quality yields, reduce or eliminate environmenta hazards associated with pesticides, and lower costs of labor and equipment needed for the latter’s application.
Among vegetable crops, onions are second only to tomatoes in world’s economics (Rabinowitch & Brewster, 1990). However, despite their long history of domestication, their global economic importance, and their impact on local agriculture, most established cultivars of these crops lack resistance to pests.
Some cultivars of the Japanese bunching onion, a crop of major economic importance in the Far East (Rabinowitch & Brewster,