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02-17-05
Leaf Spots Not Always From Pests
Gardeners are often quick to blame insects
or infectious disease when their plants develop leaf spots. But, in many
cases -- especially with houseplants -- those spots may be due to environment
and cultural practices.
House plants that experience extremes in
soil moisture often develop spots on their leaves, called "oedema"
(sometimes spelled "edema"). The spots may first appear as a
blister or raised spot, particularly on the undersides of leaves, but
may occur on the top side as well as on the stems. Eventually, the blister
develops a rust-colored, cork-like scab.
Oedema occurs when plants have been subjected
to overly dry conditions, followed by abundant moisture. The plant cells
take on too much water too rapidly, causing them to burst. Oedema can
also occur on outdoor plants. The blisters appear when soils are abundantly
moist, coupled with cloudy, humid weather.
Oedema spots do not spread, but the scab
will remain unsightly. The dead tissue may dry and fall away on thin-leaved
plants, leaving a "shot-hole" appearance.
To prevent oedema from occurring, avoid extremes
in watering practices. Feel the soil of houseplants before you water to
be sure the soil needs additional moisture. Mulching outdoor plants will
help avoid extremes in the soil moisture supply.
Gardeners often become alarmed when their
prized African violet develops leaf spots. Although there may be other
possibilities, these spots are usually just the result of splashing water.
African violet leaves are sensitive to cold water and will form spots
where the water contacts the leaf tissue. Watering from below is often
recommended as a remedy.
Placing water in the bottom dish and allowing
it to be drawn up into the soil should avoid the splashing of water onto
the leaves. However, continuous watering from the bottom can lead to a
buildup of salt residue in the soil. Most tap water has some degree of
mineral salt content, and some sources may have a considerable amount.
Fertilizing the plants will also add salts to the soil. As water evaporates
from the top of the soil, the mineral salts are left behind. This is the
white, crusty material that is often seen on houseplant soil and containers.
An occasional heavy watering from above will
help flush those salts out of the pot. About every fourth watering, apply
enough water from above so that some runs out of the drainage holes at
the bottom of the pot. Be sure to discard that excess, so that the salty
drainage water is not re-absorbed into the soil.
Spotting of African violet leaves can also
be avoided by using room. Then water can always be supplied from above,
preventing the salt buildup. You will still have to ensure that the drainage
water does not become reabsorbed. A simple way to prevent re-absorption
is to set the pots up on rocks, bricks or other support so that the bottom
of the pot is out of the reach of the drained water.
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