by Lis Friemoth
Pears and apples get it ...
But it’s one problem you don’t want
Whether you have production trees or ornamental, all Malus
(apple) and some Pyrus (pear) trees are susceptible to
scab pathogens. The best method of gaining control over
this common problem is to understand the disease and the
development cycle.
What is scab?
Scab is a fungal disease that has the potential to affect
both fruit-producing and ornamental species of plants
in the rose family. The organism that causes the majority
of problems observed in this area is Venturia inaequalis
or apple scab. Pears also can be affected, although the
scab organisms are genetically different from those affecting
apples. Specifically they are V. pirina (pear scab) and
V. nashicola (Asian pear scab).
Scab diseases all have a similar life cycle and produce
similar symptoms and results. As with all pathogens, there
are three things that need to happen at the same time in
order for them to develop and successfully complete their
life cycles. These are pathogen presence, host material
and appropriate environmental conditions. In the case of
scab, the fungal pathogens are here and ready to go every
year. If you have susceptible plant material, infection
is only as far away as the right conditions.
The scab cycle begins, in the spring, from primary fruiting
bodies (or ascospores) that spend the winter lying about
on leaf and branch debris. Although scab infection can
begin at temperatures as low as 30 degrees F., the optimal
range is between 61 to 75 degrees F., and when plenty of
air moisture is present, conditions work to help the disease
develop very quickly. As these secondary spores land and
colonize new areas, the developed lesions (colored spots)
will continue to spread additional secondary spores into
the air.
Spotting it
First, check your landscape to see if you have host plants
that may be susceptible. All host plants will vary in
degree of susceptibility and may show varying levels
of infection within a single plant structure. Knowing
not only the species, but the specific cultivar or variety,
will help you to gauge the level of expected issues.
Leaves, stems, flower buds, mature flowers and fruit all
have the potential to suffer the ill effects of scab. Think
back to previous seasons. Did the plants look cruddy or
lose their leaves midway through the summer? Did you have
fruit that was spotty and unappealing? Scab doesn’t really
look like a defined spot when it first starts out. Initially,
a leaf infection starts as a somewhat fuzzy area that,
with a bit of time, looks more like a rather indistinct
spot of grayish or olive color. It then ages into quite
distinct blackish spots that look kind of moldy. From that
point, the diseased leaves tend to become disfigured, curling
slightly, then falling off during periods of intense disease
pressure. If flower buds or flowers on fruit-producing
plants are infected, don’t expect much of a crop. If set
fruit is infected, set your expectations even lower. If
the fruit reaches maturity, it is often spotty, puckered
or otherwise disfigured and generally not appetizing in
appearance.
Minimize the impact
The need for intensive chemical solutions can be eliminated
by taking the time to research, plan for, and purchase
plants with good disease resistance. There are cultural
changes that can be made in your gardening practices that
can help reduce pressures to an acceptable level. For all
plants, good cultivar choice, proper placement, appropriate
fertility treatments, adequate water provision, removal
of diseased material and use of proper pruning techniques
all will work together toward good plant health. Healthy
plants are less likely to succumb to disease (or insect)
pressures that could be the undoing of your landscape.
Because scab is spread by spores, if the debris harboring
these pathogens is removed, there is less disease potential
to start with. Pruning to increase the air flow will keep
those spores moving — hopefully away from your yard (sorry
neighbors). Over-fertilizing, under-watering, and improper
light all create situations of stress, so adjust accordingly.
If you have given your all and your plants are still suffering,
there are fungicides out there to help. Look for quality
products available for homeowner use. Some active ingredients
that work well include: chlorothalonil, myclobutanil, and
propiconazole. Follow any and all label-mixing directions,
safety equipment and all other warnings, carefully and
closely. Use common sense and don’t hurt yourself trying
to help a plant.
Cultivars that are disease-resistant include Liberty (one
of the best), Jonafree, Priscilla, and/or Freedom for eating
apples. And for ornamental crabapples, a few of my favorites
are Louisa, Prairiefire, and Red Jade.
— Lis Friemoth is a horticulture diagnostician. Contact her at (262) 745-2904, P.O. Box 58, Springfield, WI 53176, visit online at www.thegardenhoe.com or e-mail her at gardenhoe@tds.net. Listen to Liz from 9 a.m. to 10 a.m. every third Saturday on WISN AM1130 radio. |
