FIVE EASILY CONFUSED PLANT PROBLEMS
TWO
BLIGHT OR DROUGHT STRESS?
Late blight (Phytopthora infestans) is a devastating fungal
infection whose name strikes dread into any vegetable
cultivator growing in a mild, damp climate. Once rain washes
late blight spores onto your plants, they spread voraciously,
reducing potatoes and tomatoes to rotten mush within a few
weeks. Alarm bells should start ringing as soon as you spot
tell-tale brown patches on the leaves. However, brown spots
also turn up on tomato and potato leaves after prolonged spells
of drought. Plants usually recover from drought, but blight is
always fatal.
Telling Them Apart:
At first glance, the browning caused by drought
damage and the lesions of late blight look quite similar.
But look closer and you’ll find the brown patches on
drought-stressed leaves are just dry, dead leaf. Blight
lesions, by contrast, are fuzzy with spores. They also
sometimes appear on the stem.
ONE
RED SPIDER MITE OR MAGNESIUM DEFICIENCY?
Imagine a greenhouse full of cucumbers whose leaves
gradually begin to turn yellow. At a glance, you could easily
diagnose the problem as a lack of magnesium, an essential
mineral that plants need to produce healthy leaves and
photosynthesize. However, look closer and you may notice
tiny specks clustering under the leaves and fine web-like
threads. These are the tell-tale signs of the red spider mite, a
tiny but voracious sap-sucker that attacks most greenhouse
crops from mid to late summer.
Telling Them Apart:
Red spider mites tend to cause finely mottled leaves,
while a magnesium deficiency causes leaves to be
blotchier. Also, unlike with red spider mites, the veins of
magnesium-deficient leaves stay green. To make sure
what you’re dealing with, turn the leaves over. Red spider
mites appear as tiny yellowish-green specks on the
undersides. As infections build, they also produce a fine
silk webbing much like cobwebs.
Treatment:
Red Spider Mite: Keep numbers down by releasing a
biological control such as the predatory mite Phytoseiulus
persimilis, or spray with insecticidal soap.
Magnesium Deficiency: Spray leaves with a foliar feed
made of a solution of Epsom salts two or three times
every two weeks.
54
Maximum Yield
Treatment:
Blight: Remove affected foliage to slow the spread. Pick
off tomato fruits straight away. For potatoes in which the
infection covers about a third of the plant, cut away the top
growth and harvest early.
Drought damage: Prune out affected growth and the plant
should recover. Also take the steps to improve water
delivery. A compost mulch over damp soil locks in water,
and irrigation channels alongside each row make sure
none is lost to evaporation.