Agriculture Reference
In-Depth Information
5
I
NTERACTIONS WITH
O
THER
D
ISEASES AND
S
YMBIONTS
The grower of vegetable alliums does not operate in a sterile environment, and
his crops may be assailed by a variety of diseases and pests which may slow
growth, lower yield and cause damage and disfigurement that renders them
unmarketable. Allium crops are also particularly susceptible to competition
and suppression from weeds, and the reasons for this are outlined.
The main pests and disease-causing organisms and the symptoms they can
cause are briefly described. The intention is to show how scientific knowledge of
weed, pest and disease biology underlies rational crop protection. Attention is
also paid to the interrelations between different classes of noxious organisms.
For example, allium viral diseases are vectored by pest insects and mites, pest
damage can predispose plants to infection by disease-causing bacteria and
fungi, and weeds often act as alternative hosts for such bacteria and fungi. This
chapter ends with a section on the mycorrhizal fungi of allium roots where,
refreshingly, we see an example of fungi that can be of direct benefit to the plants.
Symptoms of a number of the important pests and diseases are illustrated in
Plates 2 to 9.
There are several publications giving excellent colour illustrations of allium
pests and diseases, along with drawings and details helpful for their
identification, sometimes presented as diagnostic keys. These include the topic by
Schwartz and Mohan (2008) and the CD-ROMs of Maude and Ellis (2001) and
BCPC (2003). Excellent illustrations, along with details of biology and control
methods, are freely available at numerous web sites, particularly those of many
US land-grant universities, in particular Colorado State University, which
includes a link to many other 'onion and garlic' web sites (
http://www.
colostate.edu/orgs/VegNet/vegnet/onions.html)
and the University of California,
which gives a comprehensive coverage of weeds, pests and diseases (
http://www.
ipm.ucdavis.edu/PMG/crops-agriculture.html).
The research literature on pests,
diseases and weeds was reviewed in Rabinowitch and Brewster (1990b), and
more recent reviews of many relevant topics may be found in Rabinowitch and
