Horticulture Principles and Practices

mite). However, the most important mite in greenhouse production is the two-spotted
mite (or red spider) (Tetranychus urticae). Mites usually hide on the undersides of
leaves. The red spiders may spin unsightly webs over leaves and flowers.
6. Scale insects. Scale insects are similar to mealybugs. Some of them also secrete
honeydew and thus cause black mold growth to appear, as in mealybug attack.
They may be armored and have rubbery outer coatings or be without such a
coating and armor.
7. Slugs and snails. Slugs and snails are mollusks (which include shell animals such
as oysters). These pests chew tender seedlings and leaves. They are nocturnal in
feeding habit and thus hide during the day under stones, leaves, and other objects;
they prefer very damp environments. Slugs and snails gain access to the greenhouse
area through growing media and attachment to plants and containers.
8. Thrips. Thrips are very tiny insects. They feed on a wide variety of greenhouse
plants, usually congregating on buds, petals, or leaf axils. A gentle tap on the
hiding place causes aphids to become dislodged. During feeding, aphids scrape
the surface of the leaf, resulting in whitish streaks that may eventually turn
brown. They excrete brown droplets that eventually turn black.
9. Whiteflies. Whiteflies are also tiny insects. The greenhouse whitefly (Trialeurodes
vaporariorum) is covered with a white, waxy powder. Greenhouse plants most
affected by whiteflies include petunia, poinsettia, ageratum, chrysanthemum, and
tomato. Since they are attracted to the color yellow, sticky strips are usually
suspended over benches to trap these flies. Whiteflies also secrete honeydew.
10. Caterpillars. Caterpillars, or the worm stage of some moths, are a menace to greenhouse
production. Examples are corn earworms (which attack buds and succulent
parts of plants such as chrysanthemum), European corn borers (bore through stems),
cutworms (attack shoots), and beet armyworms (attack plants such as geranium,
chrysanthemum, and carnation).
8.20 COMMON GREENHOUSE DISEASES
8.20.1 VIRUSES
Viruses, as previously indicated, cause stunting of affected plants and discoloration of
leaves in the form of streaks, rings, or blocks. They are seldom transmitted by seed,
exceptions including tomato ring spot and tobacco ring spot, which affect geraniums.
Other greenhouse viruses include carnation mottle, carnation mosaic, chrysanthemum
stunt, and chrysanthemum mosaic.
8.20.2 BACTERIA
Few greenhouse bacterial diseases exist. Major diseases include bacterial blight of geranium
(Xanthomonas pelargonium); bacterial leaf spot of geranium and English ivy
(Xanthomonas hederae); bacterial wilt of carnation (Pseudomonas caryophylli); and
crown gall of rose, chrysanthemum, and geranium (Agrobacterium tumifasciens).
8.20.3 FUNGI
Major greenhouse fungal diseases include the following:
1. Powdery mildew. Mildew occurs under conditions of high humidity. Plants
affected by powdery mildew have a whitish, powdery growth on plant parts.
Some plants (e.g., rose) are susceptible at an early stage and thus become very
distorted. In other species, such as zinnia and dahlia, powdery mildew occurs on
older plant parts. In the latter scenario, the economic damage is blemishing,
which makes plants less aesthetically desirable and thus not usable as cut flowers.
276 Chapter 8 Principles and Methods of Disease and Pest Control