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Situation
In the spring of 1998, severe rust disease of garlic broke out in
various parts of California. While this is not the first time that
garlic rust has been reported in the state, the severity, infection
incidence, and geographic distribution of the problem in 1998 appear
to be exceptional.
In coastal areas (Monterey and San Benito counties) and in the San
Joaquin Valley, multiple fields were infected and disease incidence
reached 100% in some cases. Disease severity was high and significant
loss of foliage was experienced. Cool, rainy spring weather has
likely encouraged the disease to develop. By May, the disease continued
to spread and satisfactory control was not achieved with the few
registered fungicides available for garlic, though in most cases
these materials were applied after disease was already well established.
Rust is not a new disease for California, and it was observed in
our state at least as early as 1934. Worldwide, the fungus causing
rust on garlic has been observed on plants in the Allium or onion
family generally wherever these crops are grown.
Symptoms
Garlic rust is readily identified. Early symptoms consist of small
yellow to white flecks, streaks, and spots on leaves. As these small
areas expand, the leaf tissue covering them breaks and the orange
spores (urediospores) of the fungus become visible as pustules.
Severely infected leaves can be almost entirely covered with pustules,
resulting in complete yellowing, wilting, and drying of the leaf.
As the disease progresses, a second, darker spore type (teliospores)
may also occur on the same leaves, resulting in black pustules.
Severe rust on garlic and other Alliums can cause extensive loss
of foliage and subsequent reduction in bulb size and quality. On
infected onion and chives, symptoms consist of small (less than
1/8 inch in diameter), white to tan spots. The orange pustules often
form concentric groups on the spot periphery. Disease severity on
onion and chives is significantly less severe than on garlic
Pathogen
The garlic rust pathogen is Puccinia porri (the name
Puccinia allii is a synonym, or equivalent name).
This fungus can infect chives, garlic, Japanese bunching onion,
leek, onion, rakkyo, and shallot. The pathogen may exist in the
form of different strains or races in that a particular isolate
will infect certain plants in the onion family while at the same
time not be able to cause disease on others. However, in our inoculation
tests and field observations, we find that this garlic pathogen
can infect onion and chives. In addition, the fungus that causes
rust on asparagus, Puccinia asparagi, can infect onion,
although this apparently is not commonly seen.
Disease development
The fungus probably survives as either urediospores or teliospores,
with urediospores apparently being the more important source of
inoculum. Urediospores are windborne and can be spread long distances.
Infection and subsequent disease development are favored by cool
temperatures and high humidity. The initial source of garlic rust
inoculum has not been identified in California.
Control
At this time there are no control recommendations. Resistant garlic
cultivars have not yet been indentified. Only a few fungicide products
are currently registered for garlic, and these do not appear to
be effective against the rust. Preliminary studies (using small
plot field experiments) have identified some effective, but unregistered,
fungicides that might be useful for rust control in future seasons.
For more information please contact:
Steven T. Koike
Plant Pathology Farm Advisor
University of California Cooperative Extension
1432 Abbott Street
Salinas, California 93901
U. S. A.
e-mail: stkoike@ucdavis.edu
telephone 408-759-7350 FAX 408-758-3018
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