Biological controls

Starting with plant material grown under quarantine conditions, where diagnostic tests are made routinely to ensure that the pathogens are not present, will reduce disease problems. The next best defence a grower has in combating diseases is knowledge, which introduces an option to tackle the problem at its source using biological controls.

by Louise Labuschagne, The Real IPM Company (Kenya) Ltd.

Diseases in carnation crops can be difficult to control with conventional fungicides because of the crop’s naturally, dense canopy, which develops quickly. The penetration of sprays down to the older leaves in the base of the crop is often limited. And yet, this is exactly the zone where dead leaves and crop debris accumulates, acting as a reservoir of disease spores – billions of spores (see Important carnation diseases). If the disease life cycle in the dense lower canopy can be interrupted by establishing a biological control, such as the beneficial fungus, Trichoderma, the level of disease could be reduced. The environment is ideal for Trichoderma growth as it is protected from UV light (which can kill Trichoderma on the upper leaf surfaces) and the humidity is higher in the lower canopy (ideal for both the pathogen and the beneficial fungus). Other, mass produced biological control agents such as the beneficial bacteria, Bacillus subtilis, can contribute to the control of pathogenic bacteria such as Pseudomonas (bacterial wilt).
If biological controls such as Trichoderma and Bacillus subtilis are to be effective, they should be used routinely from propagation through to transplanting to establish a larger presence on the leaf surface before the pathogen predominates. The addition of molasses to the bio-fungicide sprays will promote the rapid growth and establishment of the beneficial microbe.

Soil borne pathogens
Some of the soil borne diseases can remain in the soil for many years and build up where poor crop rotation is implemented. It is often impossible to continue economic production without resorting to expensive and environmentally damaging chemical soil sterilisation. To avoid this, which is actually prohibited by most flower audits, some growers have resorted to growing carnations in containers. This is more expensive and does not guarantee freedom from soil diseases unless high levels of biological controls, such as Trichoderma, are used in the bags. In this case, the Trichoderma could have been used in the ground and the container growing system avoided.

There are more acceptable means of reducing levels of soil pathogens, such as bio-fumigation (incorporation of crop residues from brassica crops), or solarisation, or a combination of these methods. These techniques tend to be less effective than chemical soil sterilants and should be used in conjunction with the addition of beneficial microbes such as Trichoderma, which will then fill the biological vacuum created by this partial soil sterilisation. A low-tech method of capitalizing on bought-in Trichoderma involves adding it to farm-made compost (after the compost process has cooled down). The Trichoderma will grow on the organic matter in the compost and increase many fold before it is added to the soil. The general rule with biological controls is that they are more effective at higher application rates. The soil enrichment with microbes should be combined with treatments at propagation and the time of transplanting. Trichoderma can also be applied down drip irrigation lines. Every opportunity to get the beneficial agent established on the plant’s roots in advance of the pathogen should be taken.

Practical tips
• A film of water on the leaf surface is often required to enable the disease spore to germinate. Although it is not possible to keep surfaces dry, the grower can reduce the length of time the surface is wet by using drip irrigation, judicious ventilation and heating to prevent condensation in a greenhouse crop, or orienting the plant rows in line with the prevailing wind to improve air circulation.
• Mineral nutrition, which increases the silicon content of plants, has been recorded as improving plant leaf structure and thereby improving plant health by promoting natural defenses. The addition of rice husk as a soil amendment is one way of doing this.
• Growing plants on raised beds improves drainage and can reduce disease symptoms by avoiding stressful growing conditions, as well as preventing the mass movement of the disease spores (sometimes motile zoospores) in free water in the bed, e.g. Phytophthora has motile zoospores.

The Real IPM Company (Kenya) Ltd is based in Kenya and works with the floriculture industry throughout East Africa and Ecuador, providing training and consultancy in Integrated Pest Management (IPM). (www.realipm.com)

Fusarium in carnations.jpg and rust in carnations.jpg
Fusarium (left) and rust in carnations; the best defense a grower has in combating diseases is to know the source and how it is spread.

Important carnation diseases1
Dead leaves and crop debris are an important source of disease for several important diseases of carnations, such as:

• 2Botrytis stem rot and flower spot;
• Fusarium stem rot (Fusarium avenacearum, F. culmorum, F. Graminearum);
• Rhizoctonia stem rot (Rhizoctonia solani);
• Calyx rot (Pleospora herbarum);
• Phytophthora stem rot (Phytophthora parasitica);
• 2Alternaria leaf spot or black mould ( Alternaria dianthi);
• Septoria leaf spot (Septoria dianthi);
• Bacterial wilt (Pseudomonas caryophylli).

The other important diseases of carnations are soil borne fungal pathogens, such as:

• Verticillium wilt (Verticillium cinerescens), also known as Phialophora wilt (Phialophora cinerescens);
• Fusarium wilt (Fusarium oxysporum f. sp. dianthi);
• Fusarium stem rot (F.avenacearum, F. culmorum, F. graminearum);
• Rhizoctonia stem rot (Rhizoctonia solani);
• Phytophthora stem rot (Phytophthora parasitica);
• Pythium root rot (Pythium spp.).

Some diseases only bridge between crops on living plant material. It is important to keep these diseases out of the propagation house by making sure that the correct environmental conditions are maintained, which prevent these disease taking hold.

• 2Carnation rust (Uromyces dianthi)
• Powdery mildew (Oidium spp)

1 Detailed information relating to the correct identification of the pathogen from the symptoms can be found online at www.floracultureinternational.com June 2007 “Combating carnation diseases”.

2Botrytis, alternaria and rust all have airborne spores and care needs to be taken when planting susceptible varieties in relation to the prevailing wind or use of ventilation in greenhouse crops.

Extra information online:

Identification of important carnation diseases
Effective Integrated Pest Management of diseases starts with correct identification of the pathogen from the symptoms.

Wilts
Fusarium wilt (Fusarium oxysporum f. sp. dianthi): Seriously infected plants will wilt and die. Although the root system usually remains intact, if the vascular system is inspected a dark brown stain is obvious. In the early stages of the disease only one side of the plant appears to be wilted with yellow leaves and the top of the main shoot will grow at a right angle to the main stem. The rot in the stem dries out as it develops into a ‘shredded rot’.

Bacterial Slow Wilt or Stunt (Erwinia chrysanthemi): The lower leaves of the plant will curl, wilt and eventually die. Plants may be stunted with narrow, yellow-green leaves. Once infected, plants may recover from wilt symptoms temporarily, only to succumb again later. It may take several weeks for the disease to develop.

Bacterial wilt (Pseudomonas caryophylli): The tops of plants or individual branches may suddenly wilt and turn grey-green then yellow and die. Internode tissue may develop cracks from which bacterial slime oozes, particularly during periods of high humidity. Stems are sticky and the outer layer is easily peeled from the stem. Vascular bundles are yellowish to brown. The disease can spread very quickly.

Phialophora wilt (Phialophora cinerescens): Plants gradually wilt and leaves turn a straw colour. Although some authorities identify Phialophora wilt as a synonym of Verticillium wilt, others indicate that it is an uncommon but distinct disease. It can be distinguished from Fusarium wilt as the wilt is of the whole plant (not one-sided as Fusarium wilt may be). Again the vascular bundles are brown. There is very limited rotting of tissue even in the later, advanced stages of the disease.

Rots
Fusarium bud rot (Fusarium tricinctum): Buds which appear normal on the outside are in fact rotted internally. The disease is spread by grass mite, Pediculopsis graminum. Fluffy white fungal growth is visible inside buds.

Fusarium stem rot (Fusarium avenacearum, F. culmorum, F. graminearum): The roots and the base of the stem at the soil line will rot and the tops of plants wilt and eventually die. Examination of the rotting tissue reveals pink cushiony growths. It can be a common disease in propagation and is both soil borne and from crop debris.

Rhizoctonia stem rot (Rhizoctonia solani): Symptoms are similar to Fusarium stem rot as stems are rotten at the soil line and the entire plant will wilt and die. However the rot progresses from the outside and dark fungal strands and sclerotia can be seen with a hand-lens rather than the pink masses.

Calyx rot (Pleospora herbarum): As the name suggest, this rot starts at tip of calyx and progresses towards base. Spread of the disease is encouraged by high humidity and presence of infected crop debris. Regular harvest and removal of infected buds will slow down the disease.

Phytophthora stem rot (Phytophthora parasitica): The symptoms of Phytophthora can easily be confused with Rhizoctonia as again the stem rots at the soil line but it will not have the dark strand or sclerotia of Rhizoctonia or the pink cottony masses of Fusarium wilt. It is spread in water and good drainage is essential in it control, so high raised beds are useful.

Pythium root rot (Pythium spp.): Plants infected with Pythium are stunted and root rot is common. It is encouraged by over watering or water-logged soils. Ensure land is leveled before planting to avoid low spots in fields and irrigate judiciously.

Leaf spots
Carnation rust (Uromyces dianthi): This is one of the most common carnation diseases. It is airborne and carried over only on living plants, not crop debris – so field hygiene and rouging of plants can play a role in limiting the spread of the disease at the beginning of the infection, prior to fungicides. Small blisters or pustules of rust-red spores form on leaves.

Alternaria leaf spot and branch rot (Alternaria dianthi, A. dianthicola): This is characterised by grey-brown leaf or petal spots with purple margins. Black spore masses form in spots. Rotting which starts at the nodes can eventually girdle the stem completely. Spores are airborne and also survive in infected plants and debris. Moist conditions for 8 to 10 hours are required for infection. Provide good air circulation and keep humidity low. Irrigate in early morning. Do not wet foliage with irrigation water.

Fairy-ring leaf spot (Cladosporium echinulatum): This disease is easy to identify from the conspicuous tan spots with concentric rings. The margins of the spots may be red and dark spores form in the centre of spots. It is air borne and spreads from infected plants and crop debris. It is encouraged by high humidity during wet weather or wet leaves in greenhouses. Keeping condensation in greenhouses under control, using ventilation and heating carefully and only irrigating in the morning will help reduce this disease.

Grey mould (Botrytis cinerea): Another potentially serious airborne disease encouraged by low temperatures and high humidity. Botrytis forms dense masses of woolly grey fungal spores which form on soft, brown, decayed blossoms and can move into plant parts wherever blossoms touch them. It survives in plant debris and field sanitation plays an important role in IPM of this disease. Do not leave old, unharvested flowers in the crop.

Greasy blotch (Zygophiala jamaicensis): Greasy spots develop on leaves with radiating web-like margins. Leaves appear ‘pimpled’. It is spread from infected plants and debris and encouraged by high humidity.

Main viruses (carnations and pinks)
Mottle Faint: Leaf mottle may be present but sometimes no symptoms occur. It is a common virus in many carnation cultivars which is spread by handling the crop in normal agronomic practices and cutting knives. It is not transmitted by aphids but may be spread in drainage water. Reduce spread of mottle by disinfecting tools between blocks of plants and several times a day. Obtain virus-free plants.

Etched ring: Older leaves and stems exhibit oval or elongated rings (rarely concentric rings). Symptoms may be slight in young cuttings. There may be no obvious effect on plant vigour. It is not spread by handling plants but by aphids, so aphid control measures are very important.

Necrotic fleck: Leaves develop reddish purple necrotic flecks, streaks, or spots, but symptoms are less pronounced in low temperatures. Again the virus is spread by aphids and it is important to obtain virus-free plants and control aphids.

Ring spot: Plants with Ringspot are stunted. The young leaves may be distorted, mottled and chlorotic with small (1- 2-cm) rings; sometimes concentric. There are no known insect vectors and it is spread by cutting knives and normal handling of plants.

Vein mottle: This virus is spread by aphids and not on cutting tools. The young leaves exhibit a vein clearing, which develops into chlorotic spots and patterns that mostly follow veins. Symptoms may disappear on older leaves. Vein Mottle is more common in Sweet Williams than carnations.

Vectors of disease spores
Some diseases can be spread by pests or insect vectors such as aphids which spread many of the carnation viruses. High levels of two-spotted spider mite (Tetranychus urticae), also known as red spider mite, can also inadvertently spread spores as they have long hairs on their backs which can brush against spores and carry them around the plant. High levels of nematodes can cause wounds on roots from their feeding and provide easy entry points for soil borne diseases. Improved, integrated pest management for these pests can have a significant effect on disease control.

For example, spider mites reproduce fast and are difficult to control with insecticides because those which remain in the lower canopy always re-infect the upper canopy. Spider mites can be easily controlled with the predatory mite, Phytoseiulus persimilis, as long as they are applied after the crop canopy is touching and in sufficient numbers – sometimes over one million predatory mites per hectare in one application. Care needs to be taken to integrate the use of Phytoseiulus with compatible sprays (including compatible fungicides).