phytophthora lecture,06

PHYTOPHTHORAPHYTOPHTHORA DISEASES OF CITRUS AND DISEASES OF CITRUS AND THEIR INTEGRATED DISEASE MANAGEMENTTHEIR INTEGRATED DISEASE MANAGEMENT

R. B. GaurR. B. GaurHead, Plant Protection Coordinated Unit, Agricultural Head, Plant Protection Coordinated Unit, Agricultural Research Station, Rajasthan Agricultural University, Research Station, Rajasthan Agricultural University,

Sriganganagar-335001 (Rajasthan). Sriganganagar-335001 (Rajasthan). Phytophthora , a soil borne fungus , is one of the oldest fungi known to man. Its species cause the most serious and economically important soil borne diseases of citrus. This disease may attack in the seed bed (damping-off) nurseries ( root and crown rot) orchard (foot rot, gummosis, fibrous root rot) and on the fruit (brown rot).

DISEASE SYMPTOMSDISEASE SYMPTOMS (i) In seed bed: Damping- Off: Newly germinated seedlings are affected.

Fungus penetrates the stem just above the soil line and causes the seedling to topple.

Infected seedlings are killed rapidly when moisture is abundant and temperatures are favourable.

Plants may recover once the true leaves have emerged and the stem tissue at the soil line has matured.

Disease may also cause seed rot or pre emergence rot.

(ii) In Nurseries:(ii) In Nurseries: Root and Crown Rot:Root and Crown Rot:

Lesions may occur on the crown roots below the soil line and symptoms in the canopy develop with out obvious damage to the trunk above ground.

Scaffold root rot or crown rot below ground may occur when susceptible root stock are used.

(iii) In orchards:(iii) In orchards:Foot rot and GummosisFoot rot and Gummosis::

These are the most serious diseases caused by Phytophthora.

Foot rot results from an infection of the scion near the ground level.

The lesion may extends on stem up to 60 cm above soil level and cause rotting of crown roots in the soil.

The initial symptom appears on the bark in the form of water soaked lesion light olive brown in color later turning dark.

The fungus grows in the cambium.

In advance stages, the bark cracks, shreds in length-wise strips as it dries and abundant gum exudation follows.

This gum being water soluble disappear after heavy rain but is persistent on the trunk under dry condition.

Slowly this lesion girdle the tree trunk.

Affected trees have pale green leaves with yellow veins, a typical girdling effect.

If the lesion cease to expand or the fungus dies, the affected area is surrounded by a callus tissue.

Fibrous Root Rot:Fibrous Root Rot: Phytophthora Phytophthora spp. infect the root cortex and cause a spp. infect the root cortex and cause a decay of fibrous roots.decay of fibrous roots.

The cortex turns soft, becomes some what dis The cortex turns soft, becomes some what dis coloured, and appears water soaked.coloured, and appears water soaked.

Subsequently root system give a stringy appearance Subsequently root system give a stringy appearance with only thread like structure.with only thread like structure.

In advance stage of decline, the production of new In advance stage of decline, the production of new fibrous roots can not keep pace with root death.fibrous roots can not keep pace with root death.

The tree is unable to maintain adequate water and The tree is unable to maintain adequate water and mineral uptake.mineral uptake.

Nutrient reserves in the roots are depleted which Nutrient reserves in the roots are depleted which results in the reduction of fruit size and production, loss results in the reduction of fruit size and production, loss of leaves, and twig dieback of the canopy.of leaves, and twig dieback of the canopy.

(iv) In Groves:(iv) In Groves:

Brown Rot of Fruit:Brown Rot of Fruit: In orchards, fruits near the ground become infected when splashed with In orchards, fruits near the ground become infected when splashed with soil containing the fungus. soil containing the fungus.

If favourable condition of optimum temperature ( 75-82 If favourable condition of optimum temperature ( 75-82 ooF) and long period F) and long period of wetting (18 plus hours) persist for long, the disease spread to fruit of wetting (18 plus hours) persist for long, the disease spread to fruit throughout the canopy.throughout the canopy.

On fruits the affected area is light brown, leathery and not sunken On fruits the affected area is light brown, leathery and not sunken compared to the adjacent surface.compared to the adjacent surface.

White mycelium forms on the rind surface under humid conditions.White mycelium forms on the rind surface under humid conditions.Most of the infected fruits soon abscise.Most of the infected fruits soon abscise.

If infected fruit is packed brown rot may spread to adjacent fruits in the If infected fruit is packed brown rot may spread to adjacent fruits in the container.container.

In storage, infected fruit have a characteristic pungent, rancid odor.In storage, infected fruit have a characteristic pungent, rancid odor.

Brown rot epidemics are usually restricted to areas where rainfall coincide Brown rot epidemics are usually restricted to areas where rainfall coincide with the early stages of fruit maturity.with the early stages of fruit maturity.

CAUSAL ORGANISMCAUSAL ORGANISM

Three speciesThree species, P. citrophthora, P. parasitica,, P. citrophthora, P. parasitica, and and P. nicotianae P. nicotianae have been reported to cause citrus diseases in India.have been reported to cause citrus diseases in India.

Phytophthora citrophthora Phytophthora citrophthora :: This spp. remains active during cold This spp. remains active during cold season under the temperature range of 24 season under the temperature range of 24 ooC to 28 C to 28 ooC and C and comparatively more harmful. comparatively more harmful. P. citrophthoraP. citrophthora attacks aerial parts attacks aerial parts (brown rot) more frequently(brown rot) more frequently. . Generally do not produce Generally do not produce chlamydospores and oospores .chlamydospores and oospores .

Phytophthora parasiticaPhytophthora parasitica : : This species remains active during This species remains active during summer season under the temperature range of 28 summer season under the temperature range of 28 ooC to 32 C to 32 ooC. This C. This spp. can tolerate temperature up to 35 spp. can tolerate temperature up to 35 ooC.C.

Phytophthora nicotianaePhytophthora nicotianae:: This species cause foot rot and root rot This species cause foot rot and root rot but usually does not infect far above the ground. Produces but usually does not infect far above the ground. Produces chlamydospores and oospores in abundance.chlamydospores and oospores in abundance.

Etiology Etiology

The pathogen inhabits in soil and is a water loving fungus. It survive in soil through The pathogen inhabits in soil and is a water loving fungus. It survive in soil through small thick walled spores (chlamydospores) and oospores which can toleratre dry small thick walled spores (chlamydospores) and oospores which can toleratre dry summer conditions. summer conditions.

With onset of monsoon and in optimum temperature ( 25-32 With onset of monsoon and in optimum temperature ( 25-32 ooC), flooding or C), flooding or excessive irrigation, these spores germinate to produce a sac like structure called excessive irrigation, these spores germinate to produce a sac like structure called sporangium.sporangium.

OOne sporangium can produce up to 50 tadpole like cells called zoospores. These ne sporangium can produce up to 50 tadpole like cells called zoospores. These zoospores swim in water and are attracted to root tips and wound to cause infection.zoospores swim in water and are attracted to root tips and wound to cause infection.

These zoospores attach them selves to feeder roots and injured portion and start These zoospores attach them selves to feeder roots and injured portion and start germinating in to a thread like fungal hyphae which inter in plant tissue and ultimately germinating in to a thread like fungal hyphae which inter in plant tissue and ultimately destroy it.destroy it.

From this hyphae, a new generation of sporangia are formed with in 24 hrs of their From this hyphae, a new generation of sporangia are formed with in 24 hrs of their entrance in tissue and again liberate the zoospores to initiate new infection.entrance in tissue and again liberate the zoospores to initiate new infection.

This cycle repeats so far soil remains saturated and thus reservoir of inoculum This cycle repeats so far soil remains saturated and thus reservoir of inoculum accumulates.accumulates.

Thus zoospores are the main propogules responsible for infection and spread of Thus zoospores are the main propogules responsible for infection and spread of Phytophthora between roots or trees.Phytophthora between roots or trees.

Rapid multiplication and spread of Rapid multiplication and spread of PhytophthoraPhytophthora ( A simple ( A simple conservative calculation):conservative calculation): Assume one gram soil harbour one sporangia.Assume one gram soil harbour one sporangia.

50 zoospores are liberated from one sporangia.50 zoospores are liberated from one sporangia.

Out of 50 only 4 remain active.Out of 50 only 4 remain active.

4 active zoospores cause infection at 4 different sites.4 active zoospores cause infection at 4 different sites.

Each infection site produces one sporangium after one irrigation or shower.Each infection site produces one sporangium after one irrigation or shower.

In all 200 zoospores will be produced in 4 sites ( 50x4=200) after one irrigation.In all 200 zoospores will be produced in 4 sites ( 50x4=200) after one irrigation.

400 zoospores will be produced after 2 irrigation (200x2=400).400 zoospores will be produced after 2 irrigation (200x2=400).






25600 zoospores will be produced after 8 irrigation (12800x2=2560025600 zoospores will be produced after 8 irrigation (12800x2=25600).). This much of inoculum would be enough to destroy the complete flush of This much of inoculum would be enough to destroy the complete flush of feeder roots and to spread the disease in the surrounding area.feeder roots and to spread the disease in the surrounding area.Threshold densities have not been established, but population of less than 5 Threshold densities have not been established, but population of less than 5 propagules / cm3 have been considered insignificant.propagules / cm3 have been considered insignificant.

FACTORS FAVOURS DISEASE DEVELOPMENTFACTORS FAVOURS DISEASE DEVELOPMENT Heavy watering in orchards, especially where there is heavy or clayey soil Heavy watering in orchards, especially where there is heavy or clayey soil is very encouraging .is very encouraging . Soils with drainage restricted by hardpans or clay layers or those with Soils with drainage restricted by hardpans or clay layers or those with shallow water table that temporarily rise in root zone provide ideal condition shallow water table that temporarily rise in root zone provide ideal condition for fibrous root infection and rapid buildup of Phytophthora propagules.for fibrous root infection and rapid buildup of Phytophthora propagules. Prolonged oxygen deprivation makes roots more attractive to zoospores Prolonged oxygen deprivation makes roots more attractive to zoospores and increase infection.and increase infection. PhytophthoraPhytophthora also attacks stems if there is an injury. also attacks stems if there is an injury. A temperature of 24 A temperature of 24 ooC to 32 C to 32 ooC is the most favourable for C is the most favourable for Phytophthora Phytophthora spores to get activated.spores to get activated. Whenever the fibrous roots are put under stress or are damaged, the Whenever the fibrous roots are put under stress or are damaged, the attack is more severe.attack is more severe. Use of susceptible root stock.Use of susceptible root stock. Flood irrigation and flat bed system in nurseries.Flood irrigation and flat bed system in nurseries. Low budding ( below 15 cm).Low budding ( below 15 cm). Repeated use of same land for nursery raising.Repeated use of same land for nursery raising. Location of nurseries near old orchards.Location of nurseries near old orchards.

DISEASE MANAGEMENTDISEASE MANAGEMENT

Phytophthora in citrus plants is managed by adopting Phytophthora in citrus plants is managed by adopting Integrated Disease Management Practices.Integrated Disease Management Practices.The main components of IDM for The main components of IDM for PhytophthoraPhytophthora control control are:are:

1.1. Use of resistant root stockUse of resistant root stock::

Selection of an appropriate rootstock is the basic and Selection of an appropriate rootstock is the basic and foremost step in citriculture for long productive life of foremost step in citriculture for long productive life of plants. Final selection should be after considering the plants. Final selection should be after considering the rootstock—scion compatibility for desired horticultural rootstock—scion compatibility for desired horticultural character and other prevalent citrus diseases of the character and other prevalent citrus diseases of the area/region.area/region.

Table. 1: Response of Scion and Root stocks Table. 1: Response of Scion and Root stocks against against Phytophthora.Phytophthora.

Cultivars Response

Scions

Lemons,Limes,Sweet,oranges, Grape fruits

Very susceptible

Mandarins, Tangerines and their hybrids

Susceptible

Rootstocks

Trifoliate orange, Swingle citrumelo Resistant

Sour orange, Citrus macrophylla Moderatrely resistant

Cleoptera mandarin, Troyer and Carrizo citrange, most rough lemon selections,Rangpur,Volkamer lemon

Tolerant

Sweet orange, some rough lemon selections

Susceptible

2. 2. Cultural Practices:Cultural Practices: Being soil borne nature, once Being soil borne nature, once PhytophthoraPhytophthora enters enters in a nursery or orchard, it becomes an endemic in a nursery or orchard, it becomes an endemic problem and difficult to eradicate. “Prevention is problem and difficult to eradicate. “Prevention is better than cure” should be followed strictly.better than cure” should be followed strictly. Following practices/ techniques should be Following practices/ techniques should be followed to avoid the disease:followed to avoid the disease: Plants should be selected from Plants should be selected from PhytophthoraPhytophthora free certified nurseries with high budding (above free certified nurseries with high budding (above 9” ht.).9” ht.).

While planting, care should be taken to keep bud While planting, care should be taken to keep bud union as high as possible so that irrigation water should union as high as possible so that irrigation water should not touch the scion.not touch the scion. Soil should be kept well drained and flood irrigation Soil should be kept well drained and flood irrigation and stagnation of water for longer period in the basin and stagnation of water for longer period in the basin should be avoided.should be avoided. There should be a dry period between two irrigations There should be a dry period between two irrigations but at the same time stress should not be given to the but at the same time stress should not be given to the roots because stressed roots become vulnerable to roots because stressed roots become vulnerable to Phytophthora.Phytophthora. Frequent light irrigation should be practiced. But Frequent light irrigation should be practiced. But while deciding the time table for irrigation a farmer must while deciding the time table for irrigation a farmer must keep the local conditions in mind.keep the local conditions in mind. In well established orchard irrigation should be In well established orchard irrigation should be managed in such a manner that water do not come in managed in such a manner that water do not come in direct contact with trunk and from diseased to healthy direct contact with trunk and from diseased to healthy plant. For this, channel irrigation system should be plant. For this, channel irrigation system should be adopted. adopted.

Prolonged trunk wetting from direct emitter or Prolonged trunk wetting from direct emitter or microsprinkler irrigation should be avoided.microsprinkler irrigation should be avoided. Foot rot damage can also be reduced by Foot rot damage can also be reduced by irrigating the orchard during the morning hours irrigating the orchard during the morning hours so that the bark has the opportunity to dry during so that the bark has the opportunity to dry during the day.the day. Moisture absorbing tree wraps and buildup of Moisture absorbing tree wraps and buildup of ant colonies under any kind of tree warp should ant colonies under any kind of tree warp should be avoided. Ants feed on the trunk and main be avoided. Ants feed on the trunk and main roots and roots and PhytophthoraPhytophthora spores than infect the spores than infect the open wound.open wound. Pruning should be a regular practice in Pruning should be a regular practice in orchard for free air circulation and sun rays.orchard for free air circulation and sun rays.

Injuries to trunk and root system by farm Injuries to trunk and root system by farm operation should also be avoided. Care should operation should also be avoided. Care should be taken when tilling orchards. In facts when be taken when tilling orchards. In facts when orchard comes to the fruit bearing stage zero orchard comes to the fruit bearing stage zero tillage should be practiced and weeds should be tillage should be practiced and weeds should be controlled with weedicides.controlled with weedicides. Nursery should be away from citrus Nursery should be away from citrus orchards.orchards. To avoid contamination, nursery operation To avoid contamination, nursery operation equipments should be separate from the equipments should be separate from the orchards equipments.orchards equipments. Introduction of soil and other materials Introduction of soil and other materials infested with the pathogen should not be infested with the pathogen should not be allowed. allowed.

3. 3. Chemical and bio-control:Chemical and bio-control:

Foot rot or Gummosis:Foot rot or Gummosis:

Scrap the affected portion of trunk with sharp knife taking Scrap the affected portion of trunk with sharp knife taking care not to damage the wood before application of care not to damage the wood before application of paste.The affected portion may either be painted with paste.The affected portion may either be painted with Bordeaux paste or with Ridomil (Metalaxyl plus Bordeaux paste or with Ridomil (Metalaxyl plus mancozeb) paint (Thind mancozeb) paint (Thind et alet al. 2000). Bordeaux paste . 2000). Bordeaux paste should always be applied before onset of monsoon as should always be applied before onset of monsoon as prophylactic measure.prophylactic measure. The Ridomil paint is prepared by mixing 20 gm Ridomil The Ridomil paint is prepared by mixing 20 gm Ridomil in 1 litre of linseed oil. As per orchard scheduling the in 1 litre of linseed oil. As per orchard scheduling the painting is to be done during February and August month painting is to be done during February and August month (Gaur (Gaur et al.et al. 2004). However the trunk may be painted as 2004). However the trunk may be painted as soon as visible symptoms appear.soon as visible symptoms appear.

Fibrous Root Rot:Fibrous Root Rot:

For fibrous root rot infection in particular and other For fibrous root rot infection in particular and other Phytophthora related infections in general soil Phytophthora related infections in general soil drenching with Ridomil (metalaxyl plus mancozeb) @ drenching with Ridomil (metalaxyl plus mancozeb) @ 25 gm per tree (fully grown) mixed in 30-40 litre of 25 gm per tree (fully grown) mixed in 30-40 litre of water 2 times each during February and August at 15 water 2 times each during February and August at 15 days interval gave significant control of disease days interval gave significant control of disease ( Gaur ( Gaur et al.et al. 2004 2004)). . Since the excess use of Ridomil gives immunity to the Since the excess use of Ridomil gives immunity to the fungi soil application of fungi soil application of Trichoderma harzianumTrichoderma harzianum @ 60 @ 60 gm per tree in 30-40 litre of water 2 times each during gm per tree in 30-40 litre of water 2 times each during February and August at 15 days interval may be February and August at 15 days interval may be recommended as economically superior and eco recommended as economically superior and eco friendly approach to manage the disease ( Gaur friendly approach to manage the disease ( Gaur et et al.2004)al.2004). .

Both these treatments should be applied below Both these treatments should be applied below the canopy area. Light irrigation may be applied the canopy area. Light irrigation may be applied immediately if water used to dissolve the immediately if water used to dissolve the chemical or bioagent is less than 30-40 litre. chemical or bioagent is less than 30-40 litre. Controlled irrigation is required as the chemical Controlled irrigation is required as the chemical or bioagent should not leach more than eight or bioagent should not leach more than eight inches from the surface.inches from the surface.

Brown Root of fruit:Brown Root of fruit:

Ridomil (metalaxyl plus mancozeb) or Aliette Ridomil (metalaxyl plus mancozeb) or Aliette (Fostyl- al) may be sprayed @ 2.5 gm per litre of (Fostyl- al) may be sprayed @ 2.5 gm per litre of water in the orchard to control brown rot of fruits.water in the orchard to control brown rot of fruits.

REFERENCESREFERENCES

Davis, R.M. 1982. Control of Davis, R.M. 1982. Control of PhytophthoraPhytophthora rot and foot rot rot and foot rot of citrus with systemic fungicides metalaxyl and phosethyl of citrus with systemic fungicides metalaxyl and phosethyl alminum. alminum. Plant DisPlant Dis. 66:218-220.. 66:218-220. Gaur, R.B., M.K. Kaul, R.N. Sharma and R.R. Sharma. 2004. Gaur, R.B., M.K. Kaul, R.N. Sharma and R.R. Sharma. 2004. Integrated disease management of Integrated disease management of PhytophthoraPhytophthora rot of rot of Kinnow. Kinnow. J. Mycol. Pl. Pathol.J. Mycol. Pl. Pathol. 34 (2): 465-469. 34 (2): 465-469. Naqvi, S.A.M.H. 1994. Efficacy of some fungicides in Naqvi, S.A.M.H. 1994. Efficacy of some fungicides in control of Phytophthora disease of Nagpur mandarin in control of Phytophthora disease of Nagpur mandarin in central India. central India. Indian Phtopath.Indian Phtopath. 47:430-434. 47:430-434. Thind, S.K., J.N. Sharma, J.S. Josan and J.K. Arora. 2000. Thind, S.K., J.N. Sharma, J.S. Josan and J.K. Arora. 2000. Efficacy of various fungicidal paints on foot rot disease of Efficacy of various fungicidal paints on foot rot disease of citrus. citrus. J. Mycol. Pl. Pathol.J. Mycol. Pl. Pathol. 30:73-75. 30:73-75.

Citrus Gummosis

Citrus root & collar rot

Decline due to Phythophtora collar & root rot

Phythophtora sporangia

Phythophtora sporangium liberating zoospores

phytophthora lecture,06

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