Maximum Yield Australia/New Zealand January/February 2020 | Page 58

“
THE TROUBLE IS, yellow
leaves are the plant health
equivalent of a headache — a general
symptom that could mean anything.”
2
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.
3
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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Maximum Yield
4
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.
5
Weather
We all get a little haggard in a long winter; it’s no
coincidence that you’re more likely to fall sick in
chilly, damp conditions when you’re uncomfortable
and run down.
It’s much the same for plants, especially in these days
of increasingly dramatic changes in our climate. Extreme
weather conditions can happen any time of year and they’re
really tough for growing plants. Heavy rain can flood the soil,
drowning roots and washing out nitrogen completely. Long,
hot, dry spells lock up nutrients so roots can’t get at them. And
frosty days freeze and damage vulnerable plant cells.
All these conditions cause leaves to turn yellow, so some-
times when you’re faced with a sickly-looking plant, it’s a good
idea to look up at the sky.
Other symptoms to look out for:
• Standing water: Poorly drained soils waterlog easily, filling
soil air pockets with water and drowning roots. Digging in
organic matter helps open up heavy soils,
or you can install drainage pipes.
• Scorched leaves: Yellowing leaves with brown, scorched
tips in summer often mean your plant is gasping for
water. Water new plants particularly well in the year after
planting as they haven’t yet sent roots into the soil.
• Frost: Yellowing leaves in winter could be cold-induced
chlorosis, caused by soil microbes slowing down in cold
weather so they don’t deliver nutrients to plants. They
usually recover but covering plants with horticultural
fleece and cloches keeps them warmer longer.