Phytophthora diseases

There are a number of important species within this genus in Vietnam. These include P. nicotianae Breda de Haan on pineapples, P. palmivora E.J. Butler on durian, rubber and pineapples, P. dreschleri Tucker, P. capsici Leonian on black pepper and P. colocasiae Racib. on taro. There is potential for P. cinnamomi Rands to be a key pathogen in Vietnam, if it is not already. Some species have a very wide host range, but others are limited in their host range.

>> Pineapple heart rot
>> Gummosis of durian
>> Gummosis of citrus
>> Black stripe of rubber
>> Phytophthora blight of taro
>> Quick wilt of black pepper
>> Phytophthora cinnamomi root rot

Pineapple heart rot

Introduction

In the next 10 years, pineapple production in Vietnam is expected to increase from 35 000 ha to 100 000 ha. This increase will be largely due to the demand for the Smooth Cayenne pineapples rather than traditional varieties. The traditional variety, Queen, is already suffering losses from pineapple head rot in some regions, but since the Smooth Cayenne is highly susceptible to this disease, the large scale production of this variety is expected to lead to an increase in the incidence of Phytophthora heart rot disease. Preliminary research at Dong Giao State farm in northern Vietnam has shown the causal pathogen to be P. nicotianae although P. cinnamomi, and P. palmivora have also been isolated from disease tissue.

Disease Symptoms

Heart rot disease of pineapples is characterised by yellow-red coloured leaves that are easily pulled from the plant and white leaf bases that bear a distinct water-soaked grey-brown transverse lesion. The lesion progresses slowly from the base of the leaf to the tip.

Rotting pineapple fruit tissue may also smell badly.

If the pineapple plants develop root rot, aboveground symptoms include stunting and low yield.

Distribution within Vietnam

This disease has been recorded in all parts of Vietnam.

Host Range

The host range of P. nicotianae includes pineapple, tobacco, cocoa, cashew, jackfruit, papaya, capsicum, citrus, rubber, sweet potato, tomato, apple, cassava, black pepper, pistachio, guava, sesame, eggplant, corn and many other species.

Epidemiology

In the wet season, the disease has a tendency to be more serious. High soil pH promotes disease development, for example, in the Ninh binh and Thanh hoa provinces of Northern Vietnam. Therefore the disease is rarely found in the acid sulfate soils of the Mekong Delta.

The pathogen can be transmitted through suckers, and if pineaples are grown for two seasons, the disease incidence is likely to be very high. The pathogen can survive in the soil or in plant debris as chlamydospores for many years.

The chlamydospores germinate and hyphae infect roots or young leaf and stem tissue. The infection progresses up the root to the stem apex causing the heart rot symptoms. The pathogen can proliferate through the production of zoospores from sporangia if conditions are conducive for their production. In these situations the disease may spread through irrigation patterns.

Control

• Ensure adequate soil drainage, especially in the lower parts of the field.
• Add organic manure.
• Do not use suckers from diseased crops as planting material.
• Clear pineapple residues from the soil that are acting as mulch.
• Rotate with non-susceptible crops such as sugarcane, corn, soybean and peanuts.
• Dip the suckers in fungicides such as Aliette and Ridomil prior to planting.

Symptoms of Phytophthora heart rot 
 

Water-soaked lesion at the base of infected leaves

Pineapple and corn field 

Gummosis of durian

Introduction

This disease occurs in the southern parts of Vietnam, especially during the wet season. In 2001, the south-east region of the country had major losses of durian crops from this disease, which is the most important for durian farmers. The causal pathogen of this disease is Phytophthora palmivora.

Disease Symptoms

The first and most obvious symptom is defoliation, caused by coalescing cankers that girdle the stem or branches. Stem cankers are not always readily visible on the outer bark. If the bark is peeled away from the collar region, the wood appears watery grey, often with red streaks that intensify in colour upon exposure to air.

In the advanced stages of the disease reddish fluid exudes from cankers that dries into a rusty deposit. The pink or light brown cortical tissue darkens to brown and blackish streaks can often be seen.

Root rot, fruit rot, canker and gummosis of the trunk and branches can occur at once, causing a rapid death in 10-20 days, but a tree can survive for up to a year following infection before the leaves drop and the tree dies.

Distribution within Vietnam

This disease has been recorded in the southern parts of the country.

Host Range

The host range of P. palmivora includes durian, pineapple, cashew, breadfruit, jackfruit, capsicum, papaya, citrus, coconut, yam, taro, clove, ongan, fig, mangosteen, cotton, rubber, tomato, apple, mango, tobbacco, advocado, black pepper, guava, eggplant, potato, cocoa and vanilla.

Epidemiology

The pathogen can be spread in potting mix used to propagate seedlings and through the movement of spores in soil moisture. Infection usually results from zoospores that are produced during periods of high rainfall, so when there is excess moisture in the soil, the disease incidence is likely to be more severe. High humidity favours the disease, for example when smaller crops such as black pepper are grown beneath the canopy of the durian trees or when trees are closely planted. In dry conditions, trees may survive in infected soil because the pathogen is less active.

Control

• Prevent the movement of infected soil and water from diseased locations to clean farms.
• Pasteurise soil for seedling preparation.
• Set up an drainage system to remove any standing water beneath the trees.
• Avoid planting other crops beneath the durian trees, prevent weeds growing aroung the base of the trees and space the trees when planting to reduce the humidity and prevent disease development.
• Add organic manure to increase the biological resiliance of the soil.
• Chemical control of this disease is available by painting 2% Bordeax mixture on the trunk of the tree or injection of phosphonates. The diseased bark tissue can be removed and the exposed wood is then treated with the fungicide.
• Removing diseased fruit from the orchard floor can aid in reducing the inoculum levels.
• Select resistant rootstocks.

Canopy decline in durians

Canopy decline in durians

Canker on durian trunk

Symptoms on bark at the collar region of the durian

Durian injection for disease control 

Gummosis of citrus

Introduction

This disease occurs on all citrus trees and is caused by two Phytophthora species. P. citrophthora prefers Mediterranean type climates below 30°C, while P. parasitica (P. nicotianae) is more active in warm and humid regions above 35°C.

Disease Symptoms

Dark water-soaked regions develop on the trunk at the base of the tree, where gum can exude depending on the cultivar and climatic conditions.

In the advanced stages of the disease, diseased bark may fall off to reveal a brown stain on the diseased wood. The bark surrounding the diseased part yellows and softens. The bark then dries and cracks, and decaying inner bark can begin to smell.

The infected tree may bear a heavy crop of poor quality fruit because of the girdling effect on the phloem. The leaves yellow and drop and the tree may die , especially on highly susceptible rootstocks. The top branches of the tree may be the first to die.

Distribution within Vietnam

This disease has been recorded in all parts of Vietnam.

Host Range

Lemons and grapefruit are the most susceptible of the citrus trees, while mandarins and trifoliate oranges are highly resistant.

The host range of P citrophthora includes citrus, papaya, carambola, capsicum, chickpea, cocoa, rubber, tomato, advocado and pistachio.

The host range of P. nicotianae includes pineapple, tobacco, cocoa, cashew, jackfruit, papaya, capsicum, citrus, rubber, sweet potato, tomato, apple, cassava, black pepper, pistachio, guava, sesame, eggplant, corn and many other species.

Epidemiology

The fungus can survive in the soil for many years and needs moisture to maintain its activity. The disease was most likely spread to each of the citrus growing regions on infected rootstocks. Rain splash may disperse the soilborne pathogen to the lower parts of the tree. The disease is favoured by poor aeration beneath the tree, wet conditions and poor orchard drainage.

Control

• Select resistant rootstocks such as trifoliate rootstocks, the mandarin variety Cleopatra and the local variety 'chap'.
• Prune low hanging branches so that they are at least 60cm from the ground. Remove weeds from around the trunk to improve aeration and promote faster drying.
• Do not allow soil to cover the bud union when planting or during cultivation.
• Nursery stock should be propagated in glasshouses, on benches 35-40cm from the ground to avoid the spread of the disease into nearby stock.
• Affected tissue shoould be removed and the wound painted with fungicide. In optimal growing conditions, the wound will heal. Light pruning before spring and adequate moisture and fertiliser will encourage the trees to grow.

Black stripe of rubber

Introduction

Phytophthora palmivora and P. meadii are the causal pathogens of black stripe also known as black thread disease.

Disease Symptoms

Superficially, the symptoms can be confused with tapping injury, where sunken, discoloured areas occur on the bark just above the tapping cut. In the advanced stages of the disease, vertical fissures in the bark occur and when the bark is scraped away, black lines extend down from the tapping cut.

Distribution within Vietnam

This disease is found in the rubber growing regions of Vietnam. This disease is also found in China, Indonesia, Malaysia, Thailand, Sri Lanka and Brazil.

Host Range

The host range of P. palmivora includes durian, pineapple, cashew, breadfruit, jackfruit, capsicum, papaya, citrus, coconut, yam, taro, clove, ongan, fig, mangosteen, cotton, rubber, tomato, apple, mango, tobbacco, advocado, black pepper, guava, eggplant, potato, cocoa and vanilla.

Epidemiology

The sporangia are disseminated on moist air currents and therefore spread rapidly in light rain and wind storms. The use of infested tapping knives is also important in the dissemination of the pathogen.

Control

Dipping the tapping knives in a bleach or fungicide solution prior to cutting will limit the spread. Infected trees should not be tapped until the weather is dry. Phosphonates may provide chemical control of the disease.

Phytophthora blight of taro

Introduction

Phytophthora colocasiae is primarily a foliar pathogen with a limited host range, distributed throughout the taro growing regions of Asia on vegetatively propagated planting material and in soil.

Disease Symptoms

The leaves will develop water-soaked purple to brown circular lesions from 2 mm to 2 cm on the lobes and the sides of the leaves. The lesions gradually enlarge and become dark brown with yellow margins. White growths may be seen around the lesion and a yellow to red exudate is released from the centre of the spots. This darkens and hardens when dry.

In the advanced stages of the disease, irregularly shaped lesions partially or completely cover the leaf, and the leaves remain attached to the plant.
 
A post-harvest rot of infected rhizomes can also occur, where the pale brown, firm and dry lesions rot under poor conditions.

Geographical Distribution

The disease is found in the taro growing regions of Vietnam.

Host Range

P. colocasiae causes blight of taro (Colocasia esculenta) and yam (Alocasia spp.), rhizome crops that are staple foods for many developing countries.

Epidemiology

The sporangia are produced in wet conditions and are disseminated by rainsplash. Infection usually begin where there is water resting on the leaf, in day temperatures between 25 and 28°C, night temperatures 20-22°C and periods of rainy, overcast and humid weather. The pathogen survives in infected plant debris and rhizomes, producing oospores and chlamydospores to maintain the inoculum levels in the soil indefinately.

Control

Burn infected leaves and remove corms from the soil after harvest to prevent the overseasoning of the pathogen. To reduce the humidity and disease spread within the crop, avoid planting in shaded areas, intercrop with non-susceptible plants and increase the spacing between plants. Copper oxychloride sprays or the application of Bordeaux mixture to the foliage may be beneficial. Dipping the harvested corms in 1% sodium hypochlorite may limit post-harvest rot, so long as material is dried prior to storage.

Dark brown lesions with yellow margins 

Quick wilt of black pepper

Introduction

This disease is characterised by rapid wilting and yellowing of the plant caused by a root rot. Surveys in Vietnam have documented the isolation of Phytophthora capsici from black pepper displaying the classic symptoms of quick wilt as observed in other countries, however further research into the pathogenicity of these isolates on this host is needed.

Disease Symptoms

The disease first appears as a slight wilt of the vine. The leaves become a paler green, the vines droop, the leaves curl inward along the edges, become yellow and then drop prematurely. After leaf drop, the fruit dries. Within 7 to 14 days, most of the leaves are lost, resulting from the rotting of the main root system and the girdling of the stem at the crown region. The root infection is usually unnoticed until the leaves begin to yellow.

Geographical Distribution within Vietnam

The symptoms of this disease have been observed in parts of Vietnam. The pathogen P. capsici was isolated from diseased pepper in the Quang tri and Dac lac provinces.

The 'quick wilt' disease has caused major losses to black pepper in India and Indonesia and has also been reported in Thailand and Sarawok.

Host Range

P. capsici is pathogenic to black pepper (Piper nigrum) and betle (Piper betle L.).

Epidemiology

The pathogen survives in infected plant debris, and infection occurs throught the main roots. Leaf infection can occur after rainsplash has dispersed inoculum from the soil. The disease is more severe in the wet season, and symptoms are usually delayed if the temperature remains above 28°C. Soils low in organic matter have been shown to increase disease incidence.

Control

• Plant on well drain soils that have been free of black pepper for at least one year.
• Treat the soil with a copper based fungicide such as Bordeax mixture after removing diseased plants and debris.
• Add organic matter to the soil.

Phytophthora root rot - Phytophthora cinnamomi

Introduction

Phytophthora root rot is probably the most common cause of death of trees and shrubs. The disease is common in most parts of the world wherever there is enough water to allow the pathogen to be active and temperatures are high enough even for a short period of time. P. cinnamomi is the most important species of Phytophthora causing root rot but many other species can cause root rot diseases.

Disease Symptoms

Early signs of infection are often difficult to observe but include darkening of young feeder roots and occasionally the larger roots, reduction in the glossiness of the leaves, leaf drop and dieback. The foliage may wilt, yellow and dry up. The base of the trunk may swell and splits may form and large cankers may develop. The damage caused by the disease most often occurs or is observed in summer when plants are drought stressed. The plant is unable to adequately absorb enough water from the soil because its roots are damaged and consequently often die.

Description of the Pathogen

Phytophthora is not a true fungus, although it shares many features with the fungi. There are many species, and the differentiation between individual Phytophthora species can be extremely difficult. Although identification requires a high degree of expertise and training, it is important as species can vary significantly in their aggressiveness as pathogens. There is much debate and confusion in the characters used to define species, but general features such as colony morphology, and the production of structures such as sporangia and oospores are the basis of the taxonomy. It is likely that molecular techniques will become the best method for identification in the future.

Distribution within Vietnam

This disease is found in many parts of the world, but not yet recorded in Vietnam.

Host Range

P. cinnamomi, with one of the widest host ranges of any plant pathogen has been found in pineapple in Vietnam, but it has not yet been identified as a major pathogen. Hosts include avocado and many other trees. Many other species of Phytophthora causing root rot also have a wide host range.

Epidemiology

Soilborne species of Phytophthora require moist soil and warm temperatures to be active. They release motile zoospores which attach to and infect roots, normally behind the root tip. Phytophthora grows through the root destroying the tissue which is then unable to absorb water and nutrients. Further zoospores are produced in sporangia, particularly when the soil is moist and warm, and are released into the soil. Consequently zoospore numbers can build up quite rapidly. Zoospores move in water and may infect neighbouring plants, especially those down slope from a site of infection. These spores are easily transported in stormwater, drainage water, and contaminated soil and on tools, footwear and vehicles. A further two spore types may be produced, a chlamydospore and an oospore, which are survival structures produced when conditions become unfavourable, such as when a food source is exhausted or in periods of low temperature or drought. These spores are capable of surviving for extended periods of time, and when conditions become favourable, they germinate and renew the life cycle. This enables Phytophthora to survive in dead plant tissue for a number of years.

Control

There are now a number of control measures available for the control of Phytophthora root rot.

1. Production of disease free planting material
2. Prevention of movement of any infected material into the farm
3. Ensuring that drainage is adequate to prevent ponding/ pooling of water around roots which will enhance infection
4. Chemical control - The chemical potassium phosphonate has been shown to provide good control of this disease in many situations. The chemical has been used as a stem injection, foliar spray or as a soil drench.

Canopy decline symptomatic of Phytophthora root rot on avocado