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THE TROUBLE IS, yellow leaves are the plant health equivalent
of a headache — a general symptom that could mean anything.”
Mineral deficiency
Feeling pale, a little jaded, under the weather?
All are symptoms of anemia, or shortage of iron. It’s
the same for plants: when plants are short of the
minerals they need to thrive, they do the plant equivalent
of going pale — their leaves turn yellow.
Most soils contain a good mix of minerals, but they
aren’t always available to your plants. Acid-loving
blueberries grown in alkaline soil are unable to absorb
iron; forget to feed plants in pots and they quickly use
up the nitrogen in the potting compost and start to
starve. Waterlogging and drought can lock up minerals
away from plants, too.
Other symptoms to look out for:
• Green veins on older leaves: Yellowing leaves with
green veins could mean magnesium deficiency,
sometimes caused by over-feeding. Apply Epsom salts
as a foliar spray every two weeks.
• Brown, crispy leaf edges and green veins on young
leaves: This is iron deficiency, common in acid-loving
plants grown in alkaline or neutral soils. Grow in pots of
ericaceous compost instead.
• Spindly growth: Yellowing, weak, slow-growing plants
are often short of nitrogen. Put it right with a good feed
of nitrogen-rich liquid feed, followed up with a slow
release feed and mulch.
• Purple tints: Yellow leaves blotched with purple can
mean potassium deficiency, especially if plants aren’t
fruiting well. Remedy with a dose of potassium-rich
liquid tomato feed.
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Virus infection
Luckily for gardeners, viruses aren’t as common
in plants as they are in humans. But they
work the same way: tiny microscopic parasitic
organisms infiltrate a plant’s system cell by cell, distorting
and stunting growth and turning leaves yellow, mottled,
striped, and streaked.
There is no cure, so the only way you can deal with a virus
is to stop it from infecting your plants in the first place.
Viruses can be carried by aphids and other insects, so keep
on top of pest control; weeds also act as host plants. Dip
pruners in disinfectant between plants as a precaution, too.
Other symptoms to look out for:
• Mottling: Common in cucumber mosaic virus, which
infects cucurbits (cucumbers, squashes, and pumpkins)
and potatoes as well as many other plants.
• Crinkly leaves accompanied by mottled yellowish
patches could mean tobacco mosaic virus, which can
affect tomatoes, eggplants, and peppers. The edges of
the leaves can also dry out.
• Stunted, twisted growth: Most viruses will cause
plants to look odd — twisted, curled leaves, sometimes
streaked white, or stunted stems, plus brown patches
on fruits are all virus symptoms.
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Maximum Yield
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Fungal diseases
The world of fungi is a mysterious place. There
are billions of them, all mostly beneath your feet,
from microscopic mycorrhizal soil fungi to giant
Armillaria ostoyae, one of which is now the largest living
thing in the world; its underground mycelia covering almost
four square miles of Oregon.
Sometimes, as in powdery and downy mildews, the fungal
growth will be obvious — in this case, as a dusty coating on
the leaf. Mostly, you won’t see the fungi infecting your plants;
all you’ll notice are the symptoms, which often include yellow,
sickly-looking leaves.
Other symptoms to look out for:
• Rusty orange patches: Rusts first appear as orange spots;
affected leaves then turn yellow and die prematurely.
There’s no cure, but you can slow the spread by picking
off affected leaves.
• Wilting: If an otherwise healthy plant yellows and then
wilts, suspect Verticillium wilt, a fungal disease which
infects water-carrying vessels so plants die of thirst. Once
it’s in your greenhouse borders, you’ll have it for years;
grow in containers or grow bags of clean compost instead.
• Black spots: Yellow rose leaves with black or dark purple
spots are a sure sign of blackspot. Prune out infected
stems and pick up and dispose of fallen leaves carefully;
some types, especially older species type hip-bearing
roses like Rosa rugosa, are less susceptible.
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Pests
Plant leaves yellow when under attack from
sap-sucking insects because they are literally
having the life sucked out of them. Colonies can
number thousands of microscopic creatures, every one of them
plugged into your plants’ veins like so many leeches. It’s no
wonder they turn pale.
Turn affected leaves over and look on the undersides, as this
is where any pests will be hiding. Sometimes you’ll find them
on the shoot tips, too, where the leaves are tender and tiny bug
mouthparts don’t have to work so hard.
Other symptoms to look out for:
• Cobwebs: You’ll need a magnifying glass to spot
red spider mites, but their silk-like cobwebs are a
giveaway. Spray with insecticidal soap or release the
biological control Phytoseiulus persimilis.
• White moths: Whitefly are invisible while plants are
undisturbed, but brush the leaves and they flutter up in
clouds. Your best defence is a biological control like the
parasitic wasp Encarsia formosa.
• Wilting: This usually means damage at the root level.
Suspect cabbage root fly maggots — easily prevented
by laying a cardboard collar around seedlings —
or vine weevil grubs, especially in container-grown
plants. Tip plants out and wash the roots clean of
compost, then repot. Or prevent damage with the
biological control nematode Steinernema kraussei.