Disease management

STEM ROT OR FOOT ROT

(Phytophthora spp., Phythium aphanidrmatum, Rhizoctonia solani)

Symptoms: The disease is characterized as water soaked patches on the stem at ground level, which enlarge and girdle base of the stem. The affected tissues turn brown than black and rot. The terminal leaves turn yellow, wilt and drop. Fruits if formed also shrivel and drop off. The entire plant topples and dies because of the disintegration of parenchymatous tissue. The internal tissues look like a honey comb. The affected roots deteriorate and loose attachment with the soil

Epidemiology: Under suitable conditions the disease is able to destroy the entire plantation within one season and makes the soil unfit for planting. The disease usually appears during rainy season and the severity depends upon the intensity of rainfall coupled with temperature. The one week old plants are more susceptible than one year old. The seedlings raised in the infested soil carry the disease to the field. Such seedlings later develop stem rot under favourable condition.

Management: Seed dressing with Captan or Chlorothalonil should be done before sowing the seeds. Soil at the orchard should be well drained before planting, application of Neem cake +Trichoderma harzianum should be provided. Health nursery or healthy plants should be planted and crop rotation with non host crop should be followed.

Soil drenching with Tridemorph (Calixin 0.1%) or Metalaxyl + Mancozeb (Ridomil MZ 0.2%) or Chlorothalonil (Kavach 0.2%) at bimonthly interval provide effective control of the standing crop.

DAMPING OFF

(Pythium, Phytophthora, Rhizoctonia and Fusarium spp.)

Symptoms: Pre emergence damping off: Characterized as toppling of the growing tip before it comes out of the soil. Post emergence damping off: Seedlings show pale withering and bending symptoms near the ground level with the severe girdling of the stem tissue. In case of Phytophthora and Fusarium, root rot is also observed. Such effected seedlings suddenly topple down

Epidemiology: Seeds extracted from diseased fruits carry the primary inoculum. Soils having diseased derbies are also a source of disease. Excessive moisture/water stagnation predisposes seedlings to infection. Heavy rains during nursery stage causes heavy mortal.

Management: Seeds for raising nursery should be obtained from healthy fruits. Water stagnation and low lying areas should be avoided for nursery. Seeds should be treated with Oxycarboxin, Carbendazim SD, Captaf, Thiram @ 2 g/Kg seeds. Soil amendments with solarization, application of neem cake + Trichoderma harzianum, Dazomet, Formaldehyde should be practised. Drenching of Nursery with Chlorothalonil (Kavach 0.2%) or Oxycarboxin (Vitavax 0.1%) or Carbendazim (Bavistin 0.1%) should be done.

ANTHRACNOSE
(Colletotrichum gloeosporioides (Penz.) Penz. & Sacc.)
Symptoms: Disease can attack fruits petioles, leaves, floral parts, etc. Water soaked spots first appear as brown superficial discoloration of the skin and then develop into circular, slightly sunken areas 1-3 cm diameter. Gradually the lesions coalesce and sparse mycelial growth often appears on the margins. Under humid conditions, encrustations of salmon pink spores often arranged in a concentric pattern develop on the surface of older spots. Fruits later turn dirty brown and rot. Infection at early stages results in mummification and deformation of fruits whereas at mature stage soft rot develops. Sometimes Chocolate sunken brown lesions appear on the ripening fruits. The petioles of the lower leaves dry and are shed.
Epidemiology: Rotting of papaya fruit is favoured by 25-30 °C with high humidity and availability of free surface water.
Management: Infected leaves should be removed and destroyed. Spraying of Mancozeb (Dithane M 45 0.2%) or Chlorothalonil (Kavach 0.2%) or Carbendazim (Bavistin 0.1%) at 15 days interval provides effective control. Dipping fruits in water at 46 to 49°C for 20 minutes shortly after harvest provides control of disease under storage.

POWDERY MILDEW
(Oidium caricae (Noack,)
Symptoms: Small circular powdery patches develop on both the sides of leaves and on stem of young seedlings. These patches gradually extend, coalesce and cover the entire leaf surface. Badly infected leaves curl, dry, hang down and ultimately fall off. Young seedlings may die under severe disease attack. Sometimes in severe cases the pathogen attack fruits also.

Epidemiology: The disease is observed between June to February months with peaks during September to November. Atmospheric temperature in the range of 16 – 23 °C and relative humidity above 65 % are favourable for the disease development.

Management: The disease is effectively controlled through the spraying of wettable sulphur (Sulfex 0.3%) when atmospheric temperature is below 30°C. Application of systemic fungicides namely Tridemifon (Bayleton 0.1%) or Carbendazim (Bavistin 0.1%) or Thiophanate methyl (Topsin M or Roko 0.1%) at monthly interval is much more effective.

PHYTOPHTHORA BLIGHT
(P. Nicotianae var. parasitica (Dastur)

Symptoms: Though the main symptoms are noticed on the fruits yet stem and leaf scars are also get infected with dull colour patchy growth. These infected areas enlarged and often completely girdle the stem of young trees resulting in the wilting of the top of the plant that eventually dies. At times, particularly in the older trees the stem is not completely girdled but plants become so weak that they may be broken off with the wind. Fruits at any stage of development may become infected and hang on the tree. As the disease progresses, the fruits shrivel, turn dark brown and fall on to the ground. Such mummified fruits ultimately become brownish black, light in weight and stone like in texture .

Epidemiology: Disease is favoured by continuous rain fall and high humidity, secondary spread through raise splash and wind. Temperature 15-35 °C is favourable for infection.

Management: Avoid planting in low lying fields and heavy soils and facilitate good drainage during rainy season to avoid water logging. Apply neem cake + Trichoderma harzianum before planting. Complete removal and destructions of infected plants and fruits from the orchard at the earliest is very important. Spraying and soil drenching with protectant fungicides namely Mancozeb (Indofil Dithane M 45 0.2%) or Chlorothalonil (Kavach 0.2%) at fortnightly intervals or systemic fungicides like Melalaxyl + mancozeb (Ridomil MZ 0.2%) or Fosetyl Al (Alliette 0.2%) impart effective disease control.

ALTERNARIA LEAF BLIGHT & FRUIT SPOT
(Alternaria alternata (Fr.) Keissl)

Symptoms: Disease infect both leaves and fruit. On leaves, light brown to dark brown prominent zonate spots are produced whereas on fruits depressed, circular to oval lesions are seen. As fungus sporulates, spots turn black. Lesions remain restricted to the surface of the fruit and rotting of the flesh is not observed. Lesions may coalesce and can cover entire fruit surface. Sometimes young growing crown gets infected and the plant dies.

Epidemiology: Disease restricted to dry environments. High disease incidence (>80%) is noticed when fruits kept in cold storage (10 °C for 14 days)

Management: Bi weekly application of Zineb (Indofil Dithane Z 78 0.2%) or Propineb (Antracol 0.2%) or Chlorothalonil (Kavach 0.2%) followed by Post harvest hot water dip (48 °C for 20 min).

BLACK SPOT
(Asperisporium caricae (Speg.) Maubl.)

Symptoms: Distinct dark black spots appear on leaves which become dull and droop. Infected leaves shrivel and dry off. On fruits, black raised pustules scattered all over the fruit are produced that are confined to the rind. The infection renders the fruit unmarketable.

Epidemiology: High humidity and temperature between 15 to 25 °C are the most favourable factors or the disease.

Management: Application of Zineb (Indofil Dithane Z 78 0.2%) or Propineb (Antracol 0.2%) or Chlorothalonil (Kavach 0.2%) provides effective control.

PAPAYA LEAF CURL DISEASE
(Papaya leaf curl virus):
Symptoms: Leaf curl of papaya is characterized by severe curling, crinkling and distortion of the leaves accompanied by vein clearing and reduction in the leaf size. Leaf margins get curled downwards and inwards with dark greenish, thick veins (Fig. 52). The leaves also become leathery and brittle and the interveinal areas get raised leading to rugosity of the leaves. In some cases enations are produced on the lower surfaces of the leaves. The petioles get twisted in a zigzag manner and cluster around the main trunk. The growth of the plant is drastically reduced. Affected plants fail to flower and in case of occasional flowering the fruit set would be rare.

Cause and Spread: The virus belongs to the begomovirus genus of Geminiviridae. It is a single stranded DNA virus. It is transmitted in nature by the whitefly (Bemisia tabaci (Gennadius). This virus is not transmitted through seed and mechanically. This disease increases with a rise in temperature coupled with relative humidity. The virus mainly perpetuates on weed hosts. Warm and dry weather favors disease spread. In southern India the disease epidemics would be more during March to June where as in Northern Indian conditions epidemics would occur from June to October.

Management practices:
A. Cultural:
1. Growing of nursery under Nylon net cover (60- 80) mesh.
2. Eradication of early-infected plants and weed hosts from the field.
2. Growing two rows of border cropping with Maize, Jowar, or Bajra gives a reduction in the disease spread.
B. Chemical:
1. Soil Application of Furadon @ 1.5 Kg ai / ha at the time of sowing.
2. Prior to transplantation the seedlings should be sprayed with Acephate 1.5 g/L or Monocrotophos @ 1.5ml per liter or Dimethoate @ 2.0 ml/L.
3. Foliar Spray of Acephate @1.5g per liter followed by Spraying of Imadacloprid @ 0.3 ml/L are
effective.
4. Chemical spray followed by neem seed kernel extract @ 2% is also effective in rotation with insecticides.

PAPAYA RING SPOT DISEASE
(Papaya ring spot virus)
Papaya ring spot disease is also known as papaya mosaic, papaya distortion mosaic, mild mosaic, papaya ring spot, papaya leaf reduction, thin leaf and distortion as all the above symptoms are caused due to Papaya ring spot virus. The typical mosaic caused by potexvirus so far not found in India.

Symptoms: PRSV-P strain naturally infects papaya and cucurbits. Plants of all ages are susceptible and symptoms are generally more severe during cooler weather. The disease derives its name from the characteristic dark green sunken rings that develop on fruit of affected plants. These rings often persist as dark orange to brown markings as the fruit matures. Dark green, water-soaked streaks develop on petioles and stems. Mottle and mosaic patterns of varying severity develop on leaves that often have a ruffled appearance . One or more leaf lobes may become stunted and fruit set is markedly reduced or absent. Fruit from affected plants have poor flavor, a leathery appearance and are predisposed to fungal fruit rots.

Cause and Spread: The virus is flexuous particle of 680-760 X12nm and virus particles consists of positive sense monopartite ssRNA. PRSV is a potyvirus belonging to Potyviridae. PRSV is transmitted by several aphid species of which the following are important Aphis gossypii Glover, A. craccivora Koch, Rhopalosiphum maidis (Fitch) and Myzus persicae (Sulzer) all of which transmit the virus in a non-persistent manner. Papaya and Pumpkin are the major primary and secondary source of inoculums while rapid secondary spread can occur very fast leading to whole plantations getting totally infected. This would happen in plantations where young plants are in the vicinity of infected plants where populations of winged aphids are high. Transmission of PRSV-P is almost entirely due to transitory aphid populations, as papaya is not a preferred host for aphids and so colonies are very rarely found on plants.

Management practices: Growing of boarder crops two rows of Sesbania or castor 15 days before planting of papaya. Rouging and removal of early infected plants as when noticed. Several cultural practices have proven useful in slowing epidemics and reducing crop damage. Establishing plantations with seedling plants free of PRSV-P is essential, and new planting should be situated as far as possible from affected plantations. Plantations can be surrounded by non host crops or inter planted with other tree crops. Growing tolerant or resistant varieties is the best option. Genetically engineered resistance against PRSV has been achieved in Hawaii using Kapoho, Sunup and Rainbow cultivars. However, in India so far PRSV resistant cultivar is not available at present.

Disease Management