Disease complexes

>> Lychee decline
>> Coffee decline
>> Dry death of sugarcane

Lychee decline

Introduction

There is currently 70 000 ha of lychees in Vietnam. Lychee decline became a major disease problem in 1997, when 300 ha of lychees from the Bac giang and Hai duong provinces died and a further 1 000 ha were affected by the disease.

The fungi associated with this disease complex include Fusarium oxysporum, Fusarium solani, Rhizoctonia sp. and Pythium sp.. It is likely that nematodes also have a role in this disease complex.

Disease Symptoms

The first symptom may appear as dieback of the new shoots on the tree. The leaves initially become dull in appearance and eventually turn brown on one part of the canopy or the entire tree, depending on the degree of root infection. If only one part of the root system is necrotic, then only that side of the foliage will become brown. The tree will die with the leaves still attached. The brown part of the canopy will not have any new growth or fruit. Infected roots become black and then necrotic. There may be pink discolouration of the collar region of the trunk. Adventitious roots develop from the base of the tree along the soil surface, which can aid in the trees survival for a short time. If the infection remains localised in one part of the canopy, the tree will survive under optimal conditions.

Geographical Distribution within Vietnam

This disease has been recorded in all the lychee growing regions of northern Vietnam. It is more serious in Bac giang, Hai duong and Quang ninh provinces.

Host Range

The 'vai thieu' cultivar of lychee is the most susceptible. The 'vai lai' and the 'vai chua' cultivars are more tolerant to the disease.

Epidemiology

The warm and wet conditions during the typhoon season promotes disease development. When there is inadequate soil drainage, the disease is more prevalent, especially in those trees growing in low lying areas.

Control

• Set up a drainage system prior to planting the lychees to ensure that there is no standing water around the base of the trees.
• Prune the tree to reduce evaporation and encourage root growth to help the tree to recover quickly.
• Add organic manure to improve the biological resilience of the soil.
• Do not propagate trees from cuttings in soil from diseased areas.

Decline symptoms at the top of the tree

Decline symptoms at the top of the tree 
 

Lychee decline of part of the tree

Lychee decline of the whole tree 
 

Lychee stem

Lychee decline symptoms on roots 

Dr Vien and Dr Summerell examining a lychee tree

Injection trials for disease control 

Coffee decline

Introduction

Coffee decline, also known as leaf yellowing of coffee, is a disease caused by infection from a number of plant pathogens. It has caused major economic losses since 1996. Symptoms develop at the beginning of the dry season, when the available soil moisture falls and the damaged root system can no longer support plant growth.

A survey conducted in 1999 found that the organisms associated with this disease include Pythium vexans and other Pythium species, Fusarium oxysporum and the plant parasitic nematodes Criconomella magnifica, Rotylenchus reniformis, Pratylenchus coffeae, Helicotylenchus dihystera, Xiphinema difussum and Meloidogyne sp.

Disease Symptoms

• The older leaves of the infected plants yellow and eventually drop, with yellowing later progressing to the younger leaves.
• The yield and the quality of the coffee beans is greatly reduced on the branches showing symptoms of decline.
• The feeder roots develop root knots, symptomatic of Meloidogyne infection. These infected feeder roots become necrotic, and later the necrosis extends into the tap root. There may be pink discolouration of the collar region of the trunk.
• Adventitious roots develop from the base of the tree along the soil surface, and the plant will die at onset of the dry season.
• Young trees are susceptible to infection and can die even in the wet season.

Geographical Distribution within Vietnam

Coffee decline has been observed in all of the highland coffee growing regions, but is most serious in Nghe an, Dac lac, Lam dong and Quang tri provinces.

Host Range

The Arabica cultivars of coffee are more susceptible than the Robusta group.

Epidemiology

When the nematode population in the soil is very high, there is increased disease pressure on the trees as the nematodes provide an entry point for fungal infection.

Infection occurs through the roots during the rainy season. Young coffee trees planted in the place of old trees are more prone to the disease. This often occurs when 20 year old trees are removed and the second crop cycle begins.

The root system can be weakened during the rainy season from strong winds. The soil surrounding the base of the tree loosens and washes away. The cavity fills with water to provide conditons conducive to disease development.

Control

• When replanting the coffee field, select resistant varieties.
• Rotation with sugarcane, corn or peanuts for 3 years between coffee crops.
• Avoid damaging the root system when applying fertiliser to the base of the coffee trees.
• In the nursery, chemical control of the nematodes or fungi may result in significant reductions in diseased plants, with the combination of nematicide and fungicide being most effective.

Click here to go to Angsana Wilt, an exotic disease complex.

Symptoms on young coffee plant

Symptoms on mature coffee plant 
  

Fungal damage on coffee roots

Nematode damage on coffee roots  

Dry death of sugarcane

Introduction

This disease was first recognised in 1988 in Quang nam province. Since then, it has been found in Quang ngai and Phu yen. Only the local variety is susceptible to infection, where defined patches of the crop dies. From a survey undertaken in 2001, pathogens found associated with the disease included the fungi Fusarium subglutinans, Fusarium spp., Pythium spp., Colletotrichum spp. and the nematode Meloidogyne spp..

Disease Symptoms

The first symptom is yellowing of the flag leaf from the base to the tip. The infection proceeds to the top of the plant within two weeks, so that the entire plant is yellow. The chlorotic tissue then dries and the root system becomes black and necrotic. Root knots from Meloidogyne infection can be present. The internal parts of the stem develops a pink-red discolouration that can proceed to the top of the plant or remain localised. The stem tissue may smell like alcohol. The plant then dies, usually still upright.

Distribution within Vietnam

The disease was first reported in the central parts of the country. It is a serious problem in the Thanh binh district of the Quang nam province, but all sugarcane growing regions should be concerned about this disease.

Host Range

The local variety has been shown to be susceptible to this disease complex but modern varieties, such as Roc from Taiwan, may be more tolerant.

Epidemiology

This disease occurs on shallow sandy soils, where the drainage is poor. Warm wet conditions are conducive to disease. The disease propagules may be transmissible through surface water movement and contaminated soil on young shoots. The disease may survive to the next season on the roots from the previous crop, particularly when those roots are used as the base of the next crop, called a rattoon crop.

Control

• Remove the infected plant residue by burning or ploughing.
• Rotate with a non-susceptible crop such as pineapple or corn.
• Select tolerant or resistant varieties of sugarcane.
• Set up a drainage system for the field to prevent excess moisture in the soil profile.
• Select disease planting material.

Sugarcane leaves

Patch of diseased plants

Infected roots