SYMPTOMOLOGY

FUNGAL WILT:

1.     Fusarium wilt

2.     Verticillium wilt

1.     Fusarium Wilt:

i.                  The first symptom appears as slight vein clearing on outer younger leaflet.

ii.           Subsequently the older leaves show Epinasty caused by dropping off the petioles.

iii.        Plants infected at the seedling stage usually wilt and die soon after appear once of the first symptom.

iv.        Older plants in the field may wilt and die suddenly. If the infection is severe and if the weather is favorable for the pathogen.

v.           In older plant leaves Epinasty and vein clearing are following-

a.     Stunting of the plant.

b.     Dropping of the lower leaves.

c.      Occasional formation of adventitious roots.

d.     Wilting of leaves and young stems.

e.      Defoliation.

f.       Margin necrosis of the remaining leaves.

g.     Finally death of the plant.

a)             

                       i.             

                       ii.             

                       iii.             

                       iv.             

                       v.             

                       vi.            After these symptoms appeared only one side of the stem and progress up word until the foliage is killed and the stems died. This is called partial wilt.

                     vii.            Fruit may occasionally become infected ad then it rots and drop off without becoming spotted.

2.     Verticillium Wilt:

                         i.               Verticillium wilt is occurred worldwide but is most important in temperate region.

                       ii.               Induces wilt in lower temperature than by Fusarium.

                    iii.               Develops primarily in seedlings, which usually dies shortly after infection.

                    iv.                More common are late infection, which cause upper leaves to drop off and other leaves to develop irregular chlorotic patches that become necrotic.

                       v.               Older plants infected with Verticillium are usually stunted and the vascular tissues show characteristic discoloration.

                    vi.               Verticillium infection may result in defoliation, gradual wilting, death of successive branches, or abrupt collapse and death of entire plant.

BACTERIAL WILT:

Ralstonia solanacearum (previously Pseudomonas solanacearum)

i.                Symptoms appear as sudden wilt, infected plant die rapidly.

ii.              Older plant show wilting of younger leaf and stunting and prematurely plant die.

iii.           The vascular tissue of stem, root and tuber turn brown.

iv.           In cross section, whitish bacterial ooze or exudates is found. Bacterial pocket developed around the vascular bundle in the pith and cortex.

v.              Roots often rot and often disintegrated by the time the plants wilt permanently.

Mechanism of Bacterial Wilt:

The bacteria enter the large xylem vessel and spreading through the stem.  Along the vessel escape into the intercellular vessel of the parenchyma, cortex and pith. Bacteria dissolve the cell wall and creates cavity filled with slimy masses of bacteria and cellular debris.

Pythium SEED ROT, DAMPING OFF, ROOT ROT AND SOFT ROT:

i.             Damping off disease of seedling occurred worldwide both in tropical and temperate climate.

     ii.            The pathogen affects seed, seedlings and root of all plants.

  iii.            The greatest damage is done to the seed and seedling roots during germination, either before or after emerge.

  iv.            Crop losses vary with soil moisture, soil temperature and other factors.

     v.            Poor germination of seed or poor emergence of seedling is the results of damping off interaction in the pre-emergence stage.

  vi.            Older plants seldom killed other infected with the damping off pathogen but they developed stem and root lesion and root rot. So that their growth may be retarded and yield may be reduced.

                  

Symptoms of Damping off:

When seeds of susceptible plants are affected by the damping off fungi, they failed to germinate, become soft and mushy, and then turn brown and finally disintegrated.

There are two types of damping off.

A.   Pre-emergence damping off.

B.   Post-emergence damping off.

A.   Pre-emergence Damping off:

Young seedlings can be attacked before emergence at any point, from which the infection spreads rapidly the invaded cell attack are collapse. And the seedlings are over run by the fungus and die. This symptom is called pre-emergence damping off.

B.   Post-emergence Damping off:

Seedlings that have already emerged are usually attacked at the roots and sometimes stems at or below the soil line. The invaded areas become water soaked and discolored, and then soon collapse. The basal part of the stem is soft and thinner. As a result the seedlings fall over on soil. The fungus continuously invades the fallen seedlings which quickly weaken and die. This symptom is called post-emergence damping off.

         

Control:

A.   Pre-emergence damping off:

-         Seed treatment.

-         Soil treatment.

B.   Post-emergence damping off.

-         Bavistin

Pythium BLIGHT:

In this case, the fungus invade and kill the seedlings and create empty patches on the lawn or field.

   

Fleshy fruits or fleshy plants, soft fleshy organ and vegetables, which are present at the contact with soil. E.g. cucurbit fruits, green beans, and potatoes are sometime infected by damping off fungi during wet period. Such infections results in cottony fungus growth on the surface of the fleshy organ, while interior parts turn into a soft, watery rotten mass called leak.

Mechanism of Pythium Disease Development:

1.           Spore germ tube or sporophytic mycelium of Pythium coming in contact with the seeds, seedlings or tissue of the host plant enters by direct penetration.

2.           Pectinolytic enzyme secreted by the fungus dissolved the pectin that holds the cells together. As a result, maceration of the tissue occurs.

3.           Pectinolytic enzymes breakdown protoplast of invaded cells and in some cases they also produce cellulytic enzymes that cause complete collapse and disintegrated cell wall.

4.           As a result, infected seed and young seedlings are killed and turn into a rotten mass.

5.           When invasion of fungus is limited to the cortex of seedlings stem, the seedling may continue to live.

6.           If the invaded tissue collapsed, the seedling can’t continue to live and fall over on the soil and die.

7.           If the infection occurred when the seedling already well developed and has well thickened lignified cell, the advance of the fungus is checked at the point of infection and only small lesion developed. This is because the fungus can’t breakdown submarine and lignin.

Favorable Condition for Pythium Disease:

1.     Soil in wet condition or prolong wet condition (high soil moisture).

2.     Soil temperature usually low 10-18˚c (low temperature favors the fungal growth)

3.     High humidity (>90%RH).

4.     Excess N₂ in the soil.

5.     If the same crop is planted in same field for several consequent years.

Management of Pythium Disease:

1.    In green house disease can be controlled by soil solarization, by stream or dry heat, should be treated with CuSO₄.

2.    Seed treatment.

3.    Use of resistant variety.

4.    Use of antagonistic microorganism.

5.    In the field, seed or bulb treatment with chemical (Vitavex or Provex or Mancozeb, Dithen-M-45, Carbendagim, Bavistin)

6.    The systemic fungicide Bavistin, Ridomil or Ridomil Gold are excellent for controlling disease.

Phytophthora DISEASE:

1.     Phytophthora infestans

2.     P. cinnamoi

3.     P. palmivora

4.     P. citrophthora

5.     P. syringae

6.     P. capsici

7.     P. parasitica

Root and Stem Rot/ Collar rot:

1.     Infected plant at first show symptoms of drought and starvation and then quickly become weakened and susceptible to attack by other pathogen.

2.     It damage where the soil become more wet and the temperature fairly low. i.e. 15-23˚c.

3.     In some diseases, the collar rot of apple tree, foot rot of citrus tree, root and crown rot of cherry and black shank of tobacco, the fungus invades and kills the bark of the lower stem.

4.     Infected plants become stunted, and die within 3-10 days after infection of perennial fruit plants. In this case, few small fruits produced each year.

5.     On all host affected by Phytophthora root rot, many small, dead and necrotic brown lesion are developed on the larger root.

6.     On young and susceptible plants the whole root may decay followed by more or less rapid death of the plant.

7.     In many cases, the fungus attacks the plants at or near the soil line, where it causes water soaking and darkening lesion of the bark.

8.     In infected area enlarges and in the plant succulent and small lesion may encircle in entire system.

9.     On the larger plant, the infected darken area may be on one side of the stem and become a depressed canker below the healthy bark.

Rhizoctonia DISEASE:

1.     Damping off is most common disease. It occurs in cold (15-18˚c) and wet soil condition. But some time it can cause infection in 35˚c temperature. Very young stage of the plant may be killed before or sown after emergence. Thick fleshy seedlings such as these are legumes and the sprout of potato tuber may show noticeable brown lesion and dead tip before there kill. After the seedlings have emerged, fungus attacked stem may show water soaked lesion. That does incapable to support toe seedlings, which then fall over and die.

In older seedlings it developed elongate and reddish brown lesion. This lesion may increase length and breadth wise and girdle the base of the stem and the plant die.

2.     Seedling stem canker/ seedling blight

This disease is also called sore shine disease, most common in cotton, tobacco, and when seedling that have escaped from damping off or seedling blight phase of the disease. In this case,  the lesion appears at reddish brown, sunken canker develop that ranges from narrow to completely girdling the stem near the soil line. If the soil temperature raises the plants can grow new root and recovered from the disease.

3.     Root lesion

Root lesion may show in seedlings and mature plants. In this case reddish brown lesion usually appear just below the level but in cool weather the lesion may enlarge in all direction and completely girdle the whole base of the plant and invades the most of root, which results weakening, yellowing and sometime death of the plant.

4.     On the low lying plants such as lettuce, cabbage, cowpea leaves touching the ground and close to it are attacked at the petiole and mid ribs of the leaves and then reddish brown sunken lesion developed. From that leaves spread infections to other leaves.

5.     On fleshy succulent root, stem, bulb, corm it causes brown rotten lesion that may be superficial and extend to the root and stem.

6.     On potato tuber it cause black scurf disease in which small hard black sclerotia occur on the tuber surface and are not removed by washing. It causes rusting or russet scab. Potato tuber may become and in a cris-cross pattern.

Sclerotium Foot Rot Disease:

Sclerotium Also cause damping off, bloom blight, stem rot, root rot, cottony rot, white mold, and crown rot diseases. Seedlings are invaded by fungus quickly and then die. Plants emerge and develop some woody tissue are not invaded by fungus, but fungus grown into the cortex slowly or quickly girdle the stem, usually infection starts at soil line. At first the lower leaves and the upper leaves turn yellow or wilt or die back.

In case of succulent stem, the stem may fall over. In case of hard stem such as tomato may be wilted. Primarily fungus grow up and covering stem by lesion and white cottony mycelium depending on the weather condition.

The fungus move more rapidly downward into the roots and finally destroy the root system. White cottony mycelium is always present in or on the infected tissue and from this it grows over soil to adjacent plant and start new infection. Invaded stem, tuber and fruit tissue are pale brown and soft but not watery or infected tissue and even on the near by soil produces numerous line of sclerotium that form white in immature stage and dark brown to black at maturity.

Mechanism of Disease Development:

The fungus attack tissues directly. The fungus secrets oxalic acid, pectinolytic enzymes, cellulolytic enzymes and other enzymes. It kills and disintegrates tissues before penetrate the host. Once established in plant can grow quickly in high temperature (32-35˚c) and high moisture condition.

Sclerotium Disease	Rhizoctonia Disease
1. It favors worm condition	1.     It favors cool condition.
2.  Lesions are soft but not watery.	2.     Lesions are water soaked.
3.  Do not occur sunken lesion.	3.     Occur sunken lesion.
4.  White cottony mycelium with sclerotia produced.	4. Sclerotia produced but sometimes mycelium present or may not present.