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PakAgriFarming

Tuesday, February 05, 2013

Symptoms of Plant Diseases caused by Fungi

Posted by Shakil Shaukat On Tuesday, February 05, 2013


Fungi and fungi like organisms (FLOs) such as Pythium and Phytophthora collectively cause more diseases in the plants then do rest of the pathogen as a whole. More than 10,000 species of the fungi and FLOs are known to be pathogen to plants. The diseases caused by the fungi are so numerous and widespread that they are found all over the world and throughout all the seasons.
Cercospora leaf spots of Bauhinia variegata 
Symptom is defined as the changes brought about in the plant as a result of interaction of the plant, the pathogen and prevailing environmental conditions. Sign, compared with symptom, is the pathogen seen on the host plant.
Symptoms caused by fungal pathogens mainly differ from the symptoms by rest of the pathogens. These symptoms can be broadly categorized as following depending upon the processes involved in the appearance of disease symptoms on the host.
  1. Necrotic Symptoms
  2. Abnormal growth and development of plant tissues
  3. Other Symptoms

1. NECROTIC SYMPTOMS

These symptoms involve the death and destruction of plant tissues resulting in brittle appearance of the tissue. All the symptoms in this category involve the necrosis of the tissue from the reaction with the pathogen and result in the formation of dead cells and tissues. 
The fungal pathogen in this case when reaches the host surface secrets enzymes which kill the tissue and then digest the nutrients from the dead tissues, conversely the tissue death may also occur as a result of the hypersensitive response (HR) which is the defense mechanism of host against the establishment and spread of the pathogen on host. In any case, the symptoms produced can be either localized (at the point of infection with host) or systemic (spreading though out the foliage, or stem, or entire plant). Following symptoms involve death and destruction of host tissue as a result of pathogen infection.

a) LEAF SPOTS

These are the localized lesions produced on the leaves of the host plants as a result of pathogen infection.
Examples: Alternaria sp., Cercospora sp., Pyllachora sp..
Cercospora leafspot on Bauhinia variegata

b) SHOT HOLE

Cercospora leaf spot (with shot holes) on Plum leaf
The dead tissue on the leaves falls out leaving behind holes. This symptom is common in peaches and plums and most commonly caused by species of Cercospora and Phyllosticta etc.

c) BLIGHTS

Blights are the general and rapid destruction of the growing succulent tissues like leaves, shoots, twigs and blossoms. The blighted tissue often gives the appearance of tissue being burnt with fire.
Examples: Phytophthora sp., Cryophonectria parasiticaAscochyta sp.

Ascochyta blight lesions on chickpea leaf

d) BLASTS

Necrotic lesions are visible on the leaves, nodes, and at the base of heads (in case of rice). There is rapid browning and death of the tissues.
Example: Rice blast is caused by Pyricularia oryzae.
Symptoms of Rice blast

e) CANKERS

The localized necrotic lesions are sunken and surrounded by successive layers of cork cells.
Examples: Phytophthora sp., Nectria sp.
Typical target shaped symptoms of Nectria canker on Maple

f) SCAB

These are the localized lesions which are due to the slightly raised and cracked outer layer of the fruits, leaves or tubers etc. the cracked tissue becomes dry and corky.
Example: apple scab caused by Venturia inaequalis.
Apple scab

g) ANTHRACNOSE

It is a type of disease in which dark necrotic, sunken lesions are produced on mainly leaves, fruits and stem. The fungal pathogens produce their spores in the asexual fruiting body called acervulus. 
For example, Elsinoe veneta on raspberry and Collectotrichum spp. cause anthracnose of cotton. 
Colletotrichum capsici cause anthracnose of chillies

h) BLOTCH

These are usually large, irregular shaped spots on the surface of the plant leaves, stem or flowers. At initial stages, when new leaf arises these may appear as small red or purple colour, circular spots on upper surface and later on lower. On maturity the spots on the upper surface are usually glossy dark purple and those on the lower are chestnut brown color.
Example: Septoria tritici blotch of wheat and Cladosporium leaf blotch of peony.
Septoria tritici blotch of wheat

i) DIEBACK

It is the progressive and extensive death of the shoots and roots that starts from the tip of the shoots.
Exmaple: Lasiodiplodia theobromae cause dieback of mango
Dieback of mango

j) DECLINE

It refers to the slow and progressive decline in plant health and vigor ant the plant show abnormality in growth, symptoms of dieback also visible; leaves are small, brittle and discoloration of leaves are common symptoms.
Decline symptoms on a tree

k) PALLOR

The chlorophyll in the leaves is dissolved hence the abnormal pale coloration of the diseased area occur when pathogen is present on or inside the leaves.

l) DAMPING OFF

The young or seedlings collapse at the back due to pathogen attack before or after the germination. Older plants are seldom killed by damping off but there is definite reduction in size and growth pattern and hence yield is reduced.
Example: Pythium sp., Phytophthora sp., Rhizoctonia solani.
Damping off in tomato seedlings

m) BASAL STEM ROT

The disintegration and rottening of the stem close to the ground associated with the destruction of the cambium and vascular tissue hence the plant showing typical symptoms of wilting.
Example: Ganoderma zonatum cause basal stem rot in palms
Basal stem rot of a palm by Ganoderma (also called Ganoderma butt rot)

n) ROOT ROT

The disintegration and decay of the tissues of roots by various fungal pathogens.
Example: Phymatotrichum omnivorum cause root rot of cotton.
Cotton root rot

o) SOFT & DRY ROTS

Wet rots are caused by certain fungi which caused by disintegration of the tissues in leaves, fruits, wood, tubers etc.
Example: Rhizopus spp. cause soft rot of sweetpotato.
Soft rot of sweet potato

2. ABNORMAL GROWTH AND DEVELOPMENT OF TISSUES

These symptoms results due to the hypertrophy (increased cell size) and hyperplasia (increased number of cells) due to the interaction of chemicals produced by the pathogen and the tissues of host. The tissues show abnormal growth pattern which result in altered morphology and physiological functioning of the affected part or entire plant.

a) GALLS

Abnormal growths (swollen/raised tissues) formed by the interaction of the certain fungi on the host leaves, stems, roots or flowers.
Example: Dibotryon morbosum causes "poop gall" of choke cherry.
"Poop gall" on choke cherry

b) CLUBROOT

Gall formation or distortion take place in the roots giving the appearance of spindle or clubs.
Example: Plasmodiophora brassicae cause clubroot of crucifers.
Clubroot of a crucifer plant

c) WARTS

Hard, benign protuberances (called warty excrescences) produced on the stems or tubers and caused by fungal or viral pathogen.
Example: Synchytrium endobioticum cause potato wart disease.
Potato Wart disease

 d) POWDERY SCAB

It is totally different from simple scab. The infected tissue has brown spongy spots, which are dry and in severe conditions give appearance of warts.

Example: Sponiophora subterranea causes powdery scab of potato.
Powdery scab of potato

e) WITCHES' BROOM

In this disease, profuse branching of the twigs takes place in which the new twigs are turned upward, short, and bear small leaves giving the appearance of witches’ broom.
Example: Moniliophthora perniciosa cause witches' broom disease (WBD) in cocoa.
Witches' broom of cocoa

f) LEAF CURLING

Easily distinguishable symptoms like distortion, discoloration and curling of the leaves due to fungal pathogen (Taphrina deformans). In the early stages the leaf show red colouration, thicker and softer then normal mature leaves.
Peach leaf curl

g) ATROPHY

It is the reduction in size due to the parasitic fungi which induce hypertrophy and the growth is said to be suppressed. For example Peranospora brassicae suppresses the floral buds in Brassica sp.

3. OTHER SYMPTOMS

a) GUMMOSIS

It is the oozing out (seeping) of the amber coloured exudate from the diseased tissue which may be bark of the stem, leaves or fruits and later sets into solid mass. The plants produce gum/exudates as a defensive mechanism against the entry of pathogen into the host tissues. Different species of Botryosphaeria cause gummosis in different plants.
Gummosis of mango trunk

b) LEAF DROPPING AND FRUIT DROPPING

Dropping of leaves or fruits is also very common symptom associated with infection of any fungal pathogen. For example, Phytophthora attack on palm, Cercospora and Hemileia.

c) WILTS / VASCULAR WILTS

In case of fungal attack the vascular bundle is blocked out by the pathogen and results in the loss in turgidity and drooping of the leaves and shoots of the plant. Wilting due to pathogen attack is permanent. Most common vascular wilts are caused by Fusarium and Verticillium.
Fusarium wilt of cotton

d) RUSTS

This is a disease characterized by rusty appearance on the leaves and stems of the host plant as a result of the infection produced by the fungal pathogen belonging to the order Uredinales (rust fungi).
Black stem rust of wheat caused by Pucinia graminis tritici

e) WHITE RUST

White coloured spots of conidia spores of oomycete Albugo candida in brassica family.
Spinach white rust caused by Albugo candida

f) SMUTS

It is a disease characterized by masses of dark, powdery spores of smut fungi belonging to order Ustilaginales.
Wheat smut caused by Ustilago tritici

g) MILDEWS

Presence of whitish mycelium and fructification covering the areas on leaves, stems or fruits. If the whitish mycelium is present on the upper surface it is called Powdery Mildew and if it present on the lower surface of the leaf it will be Downy Mildew.
Bremia, Peranospora, Plasmopara,and Pseudoperanospora cause Downy mildews of dicotyledonous plants such as lettuce, tobacco, grapes and cucurbits.
Peronslerospora, Sclerophthora and Sclerospora cause Downy Mildews of monots such as corn, sorghum and sugarcane.
Downy mildew of grapes caused by Plasmopara viticola

h) ERGOT

The grains in the heads of the cereals are replaced by black or purple coloured sclerotia (ergot fruiting body) of ergot fungus Claviceps purpurea, just before the harvest.

Ergot of rye caused by Claviceps purpurea