Page 74 - ORNAMENTALES DEL TRÓPICO

74 Ornamentales del trópicO

becomes longer than normal. This is especially bothersome to owners of some houseplants,
which do not appear normal due to the etiolation.

Deficiencies and excessive amounts of minerals also may injure plants. Toxicity from ex-

cess Cu and B are particularly noteworthy. Excess Cu sprays may result in yellow spots on

leaves that can be easily confused with fungal leaf spots. Boron toxicity symptoms often include

marginal browning and development of discrete, water soaked leaf spots.

It is also important to note that plants can suffer from soluble salt injury when fertilizer

rates are generally too high. High soluble salt concentrations in the root zone can burn off

feeder roots. Young plants may wilt, suggesting damping-off problems caused by fungi. More

mature plants may exhibit leaf tip and margin burn.
Another type of common injury is phytotoxicity from pesticides sprayed directly on plants

or drift from nearby applications. This type of phytotoxicity may very well be confused with

virus diseases or mineral imbalances. There is a great range of symptoms associated with injury

from other pesticides, including bleached spots, marginal yellowing and browning of leaves,

stunted growth, stem and branch dieback, and interveinal yellowing, depending on the pesti-

cide used and the host affected.

Fungi, Bacteria, Nematodes, and Viruses

This section will concentrate on diagnosis of plant diseases caused by pathogenic microorga-

nisms-primarily fungi, bacteria, nematodes, and viruses. If one is going to diagnose plant disea-

ses effectively, it is necessary to understand the biology of these interesting organisms.

About 85% of plant diseases are caused by fungi. Although once considered to be plants,
fungi, are no longer classified as plants, but are placed in their own Kingdom, on a level with

Plants and Animals. Fungi are multi-celled and during certain stages of their life cycles, may be

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