FUNGAL DISEASES

Fungal Diseases

1. PHYTOPHTHORA LEAF BLIGHT AND FRUIT ROT
It is an important disease in those areas where fruiting coincides with the onset of monsoon rains. Due to favourable climatic conditions the disease is so severe that sometimes it causes complete defoliation of the plants. In India, disease was first time reported in1968 as Phytophthora nicotianae var. nicotianae.

Symptoms:
  • Symptoms of the disease appear as water-soaked bleached spots on any portion of the leaf (plate 1a) resulting in premature leaf fall.
  • Small water soaked spots also appear on the fruits and the flesh below the skin become soft and usually there is a distinct line of demarcation between the invaded tissue and healthy (Plate 1b).
  • Whitish mould appears on the rotten fruits under humid conditions and completely rotten fruits may fall down on the ground.
  • Symptoms also appear on collar region of adult plants as water-soaked areas with whitish growth of mycelium engirdling the collar region and the point of contact of the soil line.
  • The rot often progresses downwards to the roots in the affected plants and there is sudden drooping of leaves giving the appearance of sudden wilt.

5.1

Pathogen (s):
  • Two species of Phytophthora namely P. capsici Leon and P. nicotianae (Breda de Hann) var. nicotianae Waterhouse have been found associated with the disease.
  • In P. capsici mycelium is hyaline, branched and non-septate but few septa are found in case of old hyphae.
  • The sporangia are hyaline, ovoid to pyriform or sometimes round to lemon shaped, non-pedicillate with a predominant, hemispherical papilla at the apex.
  • The oospores are circular to spherical which are found to germinate either by germ-tubes or by stalked or sessile germ sporangia.
  • In P nicotianae var. nicotianae, the mycelium is hyaline branched and non-septate with branches at right angles.
  • The sporangia are hyaline, non-pedicellate, globose to subglobose with a prominent beak like papilla at the apex (Plate 2a).
  • The sporangia germinate by production of 15 to 20 zoospores.
  • Oospores are formed abundantly on aerial and submerged mycelium in culture.
  • The oospores are thick walled golden brown in colour (Plate 2b).

5.2

Disease cycle and epidemiology:
  • Both pathogens survive in the form of oospores in the soil as well as in infected seed (Fig.1.).
  • Fungus germinates only by means of zoospore and not by means of germ tube as in buck-eye rot pathogen of tomato.
  • Presence of abundant rainfall, high RH and warm weather are essential for initiation of this disease.
  • Heavy rains help in sporangial formation and their germination by zoospores which are splashed with spattering rains.
  • After infection, sporangia are produced on the fruit surface and are carried by wind to the adjoining fruits and foliage.
  • Temperature ranging from 22 -25o C along with high humidity (>80%) favour disease development.

5.3

Management:
  • Collect and destroy the infected leaves and fruits regularly.
  • Drainage of the field should be proper to avoid water stagnation.
  • Apply pine needles/ grass mulch on the field floor before the onset of monsoon rains.
  • Spray the crop with metalaxyl + mancozeb (0.25%) with the onset of monsoon rains followed by sprays of either mancozeb (0.25%) or copper oxychloride (0.3%) or Bordeaux mixture (4:4: 50) at 7 to 10 days interval.

2. ANTHRACNOSE AND RIPE ROT
Symptoms: The symptoms of the disease appear in two phases i.e. die-back and anthracnose and ripe fruit rot.
a) Die-back :
  • Symptoms appear as necrosis of tender twigs from the tip downwards.
  • The entire plant or branch may wither away.
  • The twigs become straw coloured in advanced stages of the disease.
  • Large numbers of black dots (acervuli of the fungus) are seen scattered all over the necrotic parts of the plants.
b) Anthracnose and ripe fruit rot:
     
  • The disease is noticed on fully mature green fruits as well as on red ripe fruits.
  • The symptoms are characterized by the appearance of small, circular, yellowish to pinkish, sunken spots on the skin of the fruits which spread in the direction of long axis (Plate-3).
  • As the fruit matures, these spots become brownish to black and severely infected fruits look straw coloured and bear numerous dots like acervuli in concentric rings.
  • The seeds produced in such fruits are discoloured and covered with mycelial mat.

5.3b

     
Pathogen:
  • The disease is caused by Colletotrichum capsici (Syd.) Butler and Bisby. (Tel: Glomerella cingulata (Stoneman) Splaud and Schrenk.
  • The mycelium of C. capsici is septate, intercellular as well as intra-cellular and aerial mycelium appears light to dark grey in colour.
  • Acervuli are round and elongated in shape (Plate-4). Setae are scattered, brown, 1-5 septate, rigid and swollen at base and acute at apex.
  • Conidiophores are short, hyaline to faintly brown, cylindrical, septate or aseptate.
  • Conidia are falcate, fusiform with acute apices and narrow truncate base and they are uni one celled, hyaline and uninucleate.

5.4

Disease cycle and epidemiology:
  • The pathogen survives both in infected plant debris and in the infected seed.
  • The fungus can survive in plant debris in the soil for at least nine months which serve as source of primary infection, whereas secondary infection takes place through wind borne conidia.
  • A temperature of 26o C and presence of free moisture or RH 100 per cent is optimum for disease development and progress.
Management:
  • Collect and burn the infected plant debris.
  • Remove and destroy solanaceous weed hosts from in and around the field.
  • Use seed from healthy fruits.
  • Treat the seeds with captan (0.3%).
  • Spray the crop with carbendazim (0.1%) or thiophanate methyl (0.1%) or combination of mancozeb (0.25%) and carbendazim (0.05%) and repeat at 10 to 14 days interval.
3. Cercospora leaf spot:
Symptoms:
  • On leaves, symptoms appear as circular spots with brown borders and light coloured, faded or grayish central part which resemble the frogeye (Plate-5), so the disease is also called as frog eye leaf spot.
  • When many spots appear on the leaves, such leaves turn yellow and fall down prematurely.
  • Symptoms also appear on the stems and petioles as elongated or irregular spots.
  • Symptoms are also observed on fruits.

5.5

Pathogen:
  • The fungus responsible for this disease is Cercospora capsici Heald & Wolf.
  • The fungus is characterized by dark brown stromata, which produce fasciculate, straight or slightly curved, continuous, simple yellowish brown conidiophores, producing polymorphous, cylindrical, fusoid 1to 3 septate, acrogeneous yellow to brown conidia broad and tapering towards the apex.
Disease cycle and epidemiology:
  • The fungus mainly perennates through the infected plant debris in the form of mycelium or stromata.
  • In areas where fruit infection occurs, the fungus can also overwinter in the form of infected seed.
  • The conidia develop on the overwintering stromata which serve as the source of primary infection.
  • These conidia are disseminated by wind and rain splashes to the leaf surface where they germinate and cause infection.
  • Disease development is favoured by high humidity ( > 95%) and temperature (20-25o C).
Management:
  • Collect and burn the infected plant debris. Follow crop rotation and maintain proper drainage in the field.
  • Use healthy seeds and treat the seeds with captan (0.3%).
  • Spray the crop with carbendazim (0.1%) or thiophanate methyl (0.1%) or combination of mancozeb (0.25%) and carbendazim (0.05%) or difenoconazole (0.03%) and repeat at 10 to 14 days interval.

4. Powdery mildew:
It is one of the important disease problems of this crop particularly in protected cultivation conditions.
Symptoms:
  • On the under surface of the leaves white to grey-coloured spots appear (Plate-6) while their corresponding upper surface exhibit yellow lesions with brown necrotic centers. Affected leaves curl upwards.
  • Premature senescence of the leaves results in defoliation.
  • Severe infection may cause die-back of the twigs or branches and stunting of plants followed by fruit drop.

5.6

Pathogen:
  • The fungus Leveillula taurica (Lev.) Arnaud (anamorph = Oidiopsis sicula scalia Syn. Oidiopsis taurica (Lev.) Arnaud) is responsible for this disease.
  • It is an obligate parasite and ectendophytic mycelium.
  • Conidiophores are long and multi-branched.
  • Conidia are dimorphic (pyriform and cylindrical) and conidia are borne singly or in short chains and their size vary according to the isolate.
Disease cycle and epidemiology:
  • The pathogen has wide host range including both cultivated and wild hosts which ensures the survival of the pathogen from one season to another.
  • Temperatures of 25-30o C favour germination of conidia.
  • High relative humidity during night than day time and temperatures < 30o C are favourable for the disease.
  • Low relative humidity i.e. < 50 per cent favours disease spread.
Management:
  • Cultural practices like wider plant spacing, sprinkler irrigation and misting in protected structures along with increased air circulation are effective in keeping the disease under check.
  • With the initiation of disease spray the crop with systemic fungicides like hexaconazole (0.05%) or carbendazim (0.1%) or difenoconazole (0.03%) and repeat at 10 to 14 days interval.
Last modified: Friday, 2 March 2012, 6:19 AM