MIC 319 FUNDAMENTALS OF

AGRICULTURAL MICROBIOLOGY

CHAPTER 2 CAUSES OF PLANT DISEASES

BY

SITI NORAZURA JAMAL 03 006/ 06 483 2132

norazura6775@ns.uitm.edu.my

OUTLINE

nutrients, pesticide exposure, environment pollution, temperature, moisture, & light.

Abiotic factors

fungi, bacteria, nematodes, parasitic plants & virus

Biotic factors

ABIOTIC FACTORS

1) Nutritional abnormalities

Often nutrient abnormalities show up as discoloration of foliage

Common discoloration symptoms include: Yellowing

Chlorosis

Two types Nutrient deficiencies

Mineral toxicity

Nutrient deficiencies Plants require several

major (N,P,Ca,Mg) and minor (iron,copper,Zn) elements for normal growth.

Deficiency or lack of any of these essential nutrients results in disease symptoms in the plant.

The symptoms in this photo occur due to a lack of molybdenum (minor plant nutrient)

Mineral Toxicity Presence of excessive

available amounts of certain minerals in the soil can lead to mineral toxicity to the plants.

The extent of injury depends on the mineral, its concentration & species of the plant.

The plant in this photo has received excess manganese, which has proved toxic and led to yellowing.

2) Pesticide Exposure

Some pesticides , if improperly used can cause serious damage to plants.

However, the most common type of chemical injury to plants is due to soil residue or spray drift of herbicides.

Examples of pesticide toxicity are: 2,4-D damage to beans and tomatoes

Glyphose (Roundup) damage to fruit trees

The common symptoms of herbicide exposure are curling and cupping

This plant was exposed to the herbicide 2,4-D

3) Environmental Pollutants

High level of fluoride in water or fluorine gas in the air can cause symptoms like this.

The damage is concentrated towards the margins of the leaf where fluorine tends to accumulate.

E.g. Air pollution

Air pollution

Certain chemicals such as O3, SO2 and NO2 are released into the air from factories, power plants and automobile exhausts.

These chemicals can accumulate in the atmosphere insufficient concentration to cause damage to plants.

Ozone damage appears in the form of chlorosis, spots and bleaching of young leaves.

This common in certain regions of the country where there is a high ozone concentration in smog

Some of the air pollutants responsible for acid rain cause damage to vegetation in certain regions.

Ozone damage on morning glory

Ozone damage or marijuana leaves

4) Extreme Weather Conditions

Extreme of weather can also lead to plant injury. Cold injury Low temperature, like frost or freeze can damage

the exposed or sensitive organs (buds, flowers, young fruits) or may kill the entire plant.

As the new branch tips began to expand, a period of cold was severe enough to kill the growing tips.

Heat injury In this case the temperature was high enough and

damage the plant tissue When plants or plant parts are exposed to high

temperature for prolonged periods, symptoms of scorching or scalding may develop.

5) High/low soil moisture

Due to excessive watering poor drainage or flooding may cause plants to turn yellow and be stunted.

Potted indoor plants may show poor development or root rots.

At the other extreme, low moisture or drought conditions can lead to poor development, wilting and death of plants.

6) high/low intensity

High light intensity is usually not a problem but low light conditions, especially for indoor plants, lead to etiolation (tissue are yellowish)

A normal and etioled shoot under sun and artificial lighting.

BIOTIC FACTORS

1)Fungi

Largest pathogen group

More than 8000 pathogenic species

Vegetative growth through production of hyphae

Reproductive via spores

Fungal Hyphae

Some examples of plant diseases caused by fungi:

brown rot of cherries, peaches and apricots; apple scab.

powdery mildew of roses, apples and other plants.

Brown rot

(Monilinia fructicola)

powdery mildew

(Triticum sp.)

2) Bacteria

About 200 pathogenic species

Can be seen with a light microscope

Simple, unicellular

Reproduce by binary fission

Some of the commonly encountered bacterial diseases are:

crown gall of rose, grape, apple, cherry and other ornamental plants

fire blight of apple and pear

Crown Gall

(Agrobacterium tumefaciens )

Fire Blight

(Erminia amylovora)

3) Viruses

Can only be seen using an electron microscope

Extremely simple-nucleic acid with a protein coat.

Reproduce by taking over host reproductive machinery

Often associated with insect vectors.

Viruses can multiply only in a living host cell and can often spread systemically throughout the infected plant.

Viruses may be transmitted from infected to healthy plants mechanically, through grafts, or contaminated propagating material; however, the most important means of spread is by insect transmission.

Viruses cause mainly two types of symptoms: mosaics and leaf curls.

Some examples: rose mosaic

potato leaf roll

Rose mosaic

(Rose Mosaic Virus) (RMV) Potato leaf roll (Polerovirus)

4) Nematodes

Parasite worms

Very complex compared to other pathogens

Usually seen only with a light microscope

Reproduce by eggs.

Most of the nematodes feed on the underground parts of the plants (roots, tubers, bulbs, etc.) causing lesions or root knots.

Some examples: Root knot nematodes on tomato, potato, beans and

many other plants.

Stubby root nematode of corn, onion.

Root knot nematodes (Meloidogyne)

Stubby root nematode (Trichodoridae)

5) Phytoplasmas

A prokaryotic organism that lacks a cell wall and survive in the phloem of plant

The helical phytoplasmas are known as spiroplasmas.

Round or elongate Usually seen only with an electron microscope Reproduce by binary fission

Some of the commonly encountered phytoplasmas diseases:

aster yellows phytoplasma on carrots, tomatoes, onions and lettuce.

Aster yellows phytoplasma