SOFT ROT AND BLACK LEG OF POTATO

PATHOGEN:
Soft rot: Erwinia carotovora pv  carotovora.
Black leg: Erwinia carotovora pv  atroseptica.

SYMPTOMS OF SOFT ROT:

1. The soft rot symptoms begin as small water soaked lesion, which enlarges rapidly in diameter and in depth.
2. The affected area becomes soft, mushy and depressed. Tissue within the affected region becomes cream coloured and slimy, disintegrating intu a mushy mass of disorganized cell.
3. The outer surface remain intact while the entire contents have changed to a turbid liquid. Cracks develop and slimy mass exudes to the surface which turns tan, grey, or dark brown in air.
4. A whole fruit or tuber may be converted to a soft watery, decayed mass within 3-5 days. Infected fruits or tubers of many plants are collapsed and then secondary bacteria grow on decomposing tissues and produce a foul odor.

WILTING

Wilt:
1. Fungal
2. Bacterial
3. Nemic
4. Sclerotial wilt/ foot rot/ soft rot/

Vascular Wilt:

They collapse the vascular system of the plant.

Two types:

1. Fungal wilt

2. Bacterial wilt e.g. Ralstonia solanacearum (Previously known as Pseudomonas solanacearum)

1.Fungal Wilt:

a.     In tropics or subtropics Fusarium wilt by F. oxysporum

b.     In temperate Verticillium wilt by V. alboatrum or V. dahlia

Formae species of Fusarium oxysporum:

Sl	Formae species	Host
1	Fusarium oxysporum f. sp. lycopersici	Tomato
2	F. o. f.sp. melongena	Brinjal
3	F. o. f.sp.conglutinans	Cucurbits
4	F. o. f.sp.cucumerinum	Cucumber
5	F. o. f.sp.cubense	Banana
6	F. o. f.sp.vasinfectum	Cotton
7	F. o. f.sp.dianthi	Carnation
8	F. o. f.sp.melonis	Muskmelon
9	F. o. f.sp.niveum	Watermelon
10	F. o. f.sp.tuberosi	Potato
11	F. o. f.sp.pisi	Pea
12	F. o. f.sp.lini	Flax
13	F. o. f.sp. batatas	Sweet potato
14	F. o. f.sp.tracheiphilum	Soybean, cowpea

Symptoms of Fusarium wilt:

1.     Epinasty

2.     Wilting phase

3.     Yellowing phase

4.     Vein clearing

5.     Drying

Development of Fusarium wilt:

Stage 1: The leaves start dropping out at the margin.

Stage 2: Total leaves drop out

Stage 3: Leaves become yellow

Stage 4: Chlorophylls start to degrade and the veins become clear.

Stage 5: The entire plant becomes dried.

Some important features about Fusarium wilt:

1.     Fusarium wilt occurs more at reproductive stage than vegetative stage.

2.     Partial wilting is observed.

3.     Partial wilting depends upon movement of the toxin secreted by Fusarium.

4.     Chocolate or deep brown discoloration is seen in the vascular system after transverse cut. (The color is light brown in case of Verticillium wilt.)

2. Bacterial Wilt:

Symptoms of bacterial wilt:

1.     Normally vegetables are affected by the bacterial wilt.

2.     It is occurred more in early vegetative stage.

3.     Total foliage wilts completely and die within 2-3 days.

4.     Light brown discoloration found in the vascular system when cut transverse.

5.     This discoloration is occurred due to bacterial ooze, cell etc.

6.     Bacterial ooze is secreted from the affected portion.

7.     If an affected cut portion is kept in a beaker of water, the water will be dizzy.

Mechanism of Wilting:

Water and minerals pass upward through xylem vessel. This is also called water conducting vessel. Bacteria collapse this vessel so that water can not pass upward. Water deficiency is occurred. As a result for the lack of water wilt is happened.

Bacteria secrets enzymes in the affected plants, which results in opened stomata. The stomata remain open all the time and never closed. Water is lost through stomatal opening. For the excess dehydration wilt can be occurred.

MECHANISMS OF WILTING:

1.     Presence of pathogen within the vascular bundle.

2.     Activity of pathogen in the within the vascular bundle.

1.     Presence of Pathogen in the Vascular Bundle:

Fungus- mycelium, hypha, conidia,

Bacteria- bacterial cell, colony, ooze.

2.     Activity of Pathogen:

 i.      Production of polysaccharide in xylem vessel.

ii.      Production of gum and gell.

     Pathogen secrets enzyme, which breaks down the cell wall. Substances of cell wall enters into         the intercellular space and then produce gum and gell like substances.

iii. Excessive division of parenchyma tissue.

    For the presence of enzyme and toxins, parenchyma cells devide excessively and intercellular         spaces become collapse.

iv. Formation of tylases in the vascular bundle.

    Balloon shaped overgrowth in the parenchyma cell is called tyalase. For the presence of enzyme     and toxins, tyalase is produced. Toxins are Fumosin-B1, Fusario toxin, Nivalenol, etc.

v.  Movement of toxins in the leaves, resulted vein clearing  and disruption of cell permeability.

EARLY BLIGHT OF POTATO AND TOMATO

SYMPTOMS:

Symptoms On Leaf:

1. Foliar symptoms of early blight first appear as small, irregular to circular dark brown to black spots on the lower (older) leaves. These may range in size from a pinpoint to an eighth of an inch in diameter.                                                                
2. As the spots enlarge, they become restricted by leaf veins and take on an angular shape. Early in the growing season, lesions on young, fully expanded succulent leaves may be larger up to half an inch in diameter and may, due to their size, be confused with late blight lesions. 
3. Leaf lesions are relatively easy to identify in the field. Because a series of dark concentric rings alternating with bands of light tan tissue are developed giving the spots a target like appearance.                                               A narrow band of chlorotic tissue often surrounds each lesion and extensive chlorosis of infected foliage develops over time. Elongated superficial brown or black lesions may also form on stems and petioles.
4. By the end of the growing season, the upper leaves of infected potato plants may be peppered with numerous small early blight lesions .                            
5. Subsequently lesions may coalesce to cover a large area of the leaf (Fig. 5a). Severely infected leaves eventually wither and die but usually remain attached to the plant (Fig. 5b). 

Severe infection of foliage by the early to mid-bulking period can result in smaller tubers, yield loss and lower tuber dry matter content.


Symptoms On Stem:

1. Dark sunken spots develop on branches and stems of plants such as tomato. Some lesions may develop on seedling stage and form cankers, which may enlarge, griddle the stem and kill the plant.



Lesion on potato stem





Lesion on tomato stem

Symptoms on Tuber (Potato):

1. On the tubers dark, slightly sunken lesions develop that may be up to 2 cm in diameter and 5-6 mm in depth.                                                        
2. Rotting and formation of undersized and reduced number of tuber is reported to this disease.                                                                

Symptoms on the Fruit:

1. Alternaria may attack during the maturity period. the spots may be small and sunken or may enlarge to cover the most of fruit. The fruits may be leathery and have a black, velvety surface layer of fungal growth and spores. Sometimes a small lesion at the surface may indicate an extensive spread of the infection inside the fruit.   

HOST: Potato, tomato, chili, Solanum nigrum etc.

PERPETUATION OF THE CAUSAL ORGANISM/SOURCE OF INFECTION:

Primary Source:

Alternaria survive as mycelium or conidia in diseased plant debris and in or on seeds which cause primary infection in the next crop.

Secondary Source:

Infected crop.

FAVORABLE CONDITION:

• Heavy dews and frequent rains are suitable for abundant conidia production.
• Weak and wounded plants.
• Mg ion deficient field is favorable for disease.

Red Rust of Tea

Causal organism:

1. Cephaleuros parasiticus Karst

2. Cephaleuros mycoidea Karst

Type of organism: Algae.

Introduction:

This is an algal disease, common on tea and many other plants including coffee, guava, citrus and other cultivated and jungle tree. Usually this is epiphytic on the leaves and may also become parasitic. This disease is found in most tea garden in the country. This disease spread rapidly during humid weather accompanied by bright sun.

Key words: algal disease, epiphytic, parasitic, humid weather, bright sun.

Symptom:

1. The algal appears as minute rusty spots on the leaves.

Fig: A leaf of tea showing minute rusty spots on it

2. Spots enlarge, become roundish and increasing to about 10-15 mm in diameter.

3. Spots are reddish and hairy occurring on the upper leaf surface.

4. Damage is caused by reducing the effective area of photosynthesis.

Fig: Leaf of tea showing severely infected red rust symptom

5. Algae enter leaf through epidermis and take nutrient as parasite, and then the plants are severely affected.

6. Severely infected leaves are discolored.

7. Spots spread to stem.

8. Infected stems become pale and appear unhealthy even from a distance.

Disease cycle:

The disease is more or less chronic. Algae can survive in the stem and old leaves all the year round. Become severe when bright sunlight is available and also when new flush comes out. The secondary spread is through wind-borne inoculums.

Key words: stem and old leaves, bright sunlight, new flush, wind borne.

Mode of infection:

The algae forming sporangia on infected leaves and stem, which are exposed to the air. The algal cells are impregnated with a red pigment, which mask the green chlorophyll, and hence the red color.

Orange colored zoospores are formed inside the sporangia. They are round to ovoid and biflagellate for swimming in water. They germinate to form the characteristic thallus.

Key words: sporangia, red pigment, masking chlorophyll, red color, orange zoospore, round to ovoid, biflagellate, thallus.

Control measure:

It is very difficult to eradicate the disease after establishment. Regulating the overhead shed and pruning and plucking of infested leaves and shoots may help in reducing the damage caused by this alga.

Key words: overhead shed, pruning and plucking.