Control of Phytophthora, Pythium and Rhizoctonia in Container-Grown Hardy Ornamentals

Factsheet 16/04 Horticultural Bradbourne House Development East Malling Council Kent ME19 6DZ Hardy Nursery Stock T: 01732 848383 F: 01732 848498 E: [email protected]

Control of Phytophthora, Pythium and Rhizoctonia in container-grown hardy ornamentals

By Tim O’Neill and David Ann, ADAS

Root, crown and basal stem rots caused by species of Phytophthora, Pythium and Rhizoctonia are economically important diseases of container-grown hardy ornamentals. This factsheet describes the symptoms caused by these pathogens, their biology, and a management strategy for disease control. It does not deal with the diseases caused by Phytophthora ramorum (see factsheet 19/03), or with the less common crown and root rot diseases caused by other fungal pathogens.

Action points Nursery hygiene because of the high humidity. • Implement a nursery hygiene regime If necessary, use a protectant Stockplant management that covers all the stages of propa- fungicide treatment. • Ensure stock plants are free from gation and production to include Phytophthora, Pythium and cleaning and/or disinfecting beds, • Where there is significant disease Rhizoctonia and take appropriate pots, tools, machinery, water and risk, include appropriate fungicide cultural and other measures to irrigation equipment. Consider treatment(s). Early fungicide maintain their health (see provision creating a plant quarantine area treatment, before the disease is of healthy propagating material). (see section on hygiene). established, is important for effective control. • If possible, grow stock plants in Use of fungicides containers, in well drained growing • Cuttings are particularly susceptible media with high air capacity. to Rhizoctonia during rooting

• Where feasible, train stock plants to discourage prostrate growth and stimulate production of cutting material higher up.

Optimal root growth • Ensure that cultural conditions are optimal for good root growth. In particular, manage watering so that the growing media is neither too wet nor too dry, preferably using trickle or sub-irrigation.

• Avoid high conductivity, incorrect pH or nutrient imbalance in the growing media, or other factors that may check root growth, as these may encourage Phytophthora and Pythium.

• Monitor root health with regular visual checks and, where necessary, accurate expert diagnosis.

1 Stock plants grown in containers with trickle or sub-irrigation will minimise disease risk Background sometimes can also be severely crown or stem base. Infection may affected. HDC project PC 97a revealed occur on both young and mature plants. A wide range of container-grown pathogenic Pythium species to be Pythium and Phytophthora species, hardy ornamentals are susceptible widespread on many UK nurseries sometimes known as water moulds, are to root rot caused by species of growing pot or bedding plants and the closely related. For example, both are Phytophthora, Pythium and situation with hardy nursery stock exacerbated by abundant moisture, are Rhizoctonia. Infection arising during production is likely to be similar. similar in their sensitivity to fungicides propagation can lead to widespread The damage caused by and share other common features. losses. Phytophthora species is often much Rhizoctonia solani has a wide host Pythium species are probably the more severe than that caused by range and causes damping-off, root rot, most common and are particularly Pythium species, with larger as well wire-stem, crown rot and occasionally damaging during propagation and on as smaller roots being attacked and a leaf and stem blight. Mature plants young plants, although mature plants rotting sometimes extending into the are generally less susceptible.

Symptoms sloughed off to expose the central also occur on container grown stock vascular column (the stele). Large as under protection, mainly where Symptoms caused by species of well as fine roots are frequently overhead sprinklers are used. Phytophthora, Pythium and affected. The first evidence of infection is a Rhizoctonia are not always sufficiently Sometimes the fungus progresses small, grey or brown water-soaked distinctive to enable reliable visual into the stem base, causing a reddish area on expanding leaves and young diagnosis to be made. Accurate brown discolouration of the wood, shoots. These areas then expand until diagnosis is important for appropriate and wilting and withering of shoots entire leaves are destroyed and stems fungicide selection. Therefore, seek or even the entire aerial parts of the become affected and die back. expert help where there is doubt. plant. Root and stem base rot caused Notable nursery stock subjects so by various Phytophthora species is affected include Ceanothus and Ilex, Phytophthora root, crown and basal common on a range of hardy orna- both of which are widely grown in stem rot mentals. Table 1 overleaf lists suscep- containers outdoors and under protec- The effect of root rotting is often first tible species commonly grown in tion. Wounded tissue created by evident from symptoms on aerial the UK. pruning cuts or physical damage is parts of the plant, namely poor also a common entry point for infection. growth, leaf yellowing, symptoms of Phytophthora leaf and shoot blight nutrient deficiency (pale or off-colour Occasionally, leaves and shoots may Pythium root and stem base rot foliage), wilting or a rapid collapse of be directly attacked by Phytophthora Symptoms induced on the aerial parts the plant. Close examination of the species spread by splash from of the plant by Pythium root rot are root system will reveal some or most growing media or plant material similar to those induced by infection roots to be water-soaked, brown or contaminated with the fungus. This of roots by Phytophthora species (see black in colour and soft. The root is most likely to happen during heavy opposite). Close examination of the cortex (outer sheath) is easily rain in outdoor production, but can root system will reveal many or most of the roots to be discoloured and rotting. Rotting may sometimes be confined to root tips or thin lateral roots. Typically, affected roots appear water-soaked, grey or brown and shrivelled. The cortex sometimes falls away easily, leaving just the central stele. When such symptoms are exten- sive, they seriously impede root action. In non-woody species, the fungus may extend into the stem causing a basal stem or crown rot. Occasionally, Pythium species cause a basal stem rot in the absence of obvious root rot. Seedlings and cuttings affected by Pythium species collapse (damping- off) or in the case of seedlings, they may fail to emerge. Rot caused by Pythium which results in damping-off, usually progresses back from the 2 The effect of root rotting is often first evident as leaf yellowing and poor growth, as here with root tips, whereas that caused by Phytophthora cryptogea on Hebe pinguifolia ‘Pagei’ (affected plant on the right) Rhizoctonia (see opposite) usually 3 Pythium root rot in Penstemon, resulting in loss of fine roots (left) and brown spots on larger roots (right)

develops on roots and the stem base On cuttings, infection results in a dry, Root rotting caused by R. solani at or just below the compost surface. brown stem lesion progressing typically shows as discrete brown It is possible that seedlings and inwards. Sometimes strands of lesions, consisting of rotting of the cuttings only slightly affected may go the fungus are visible, growing over outer root tissues, scattered along unnoticed and be potted on, and the affected tissues and across the the length of the root. New root growing plants will then be unthrifty. surface of the propagation medium. growth may be inhibited. These root The damaged tissues may subse- symptoms are less common than Rhizoctonia root and stem base rot quently be colonised by the grey those of stem and crown rotting. R. solani is the principal Rhizoctonia mould fungus, Botrytis cinerea. species and is commonly found on On established woody plants, the Rhizoctonia web blight and leaf rot nurseries affecting a very wide range most common stem symptom is a Under warm, humid conditions, of ornamental plants. Susceptible brown sunken lesion at or just above strands of R. solani may be seen as a species are listed in Table 2. The the compost surface. Sometimes webbing growing over the leaves, fungus causes damping-off of these lesions do not penetrate the which subsequently rot. This symptom seedlings and cuttings, root rotting, stem deeply enough to seriously is known as web-blight and Calluna crown rotting and occasionally a affect growth but they can split and cuttings are particularly vulnerable. leaf and stem blight. become cankerous. Occasionally, during heavy rain, on On seedlings and young plants, the Crown and stem base rotting may plants growing outdoors, leaf stem base is often discoloured and result either from progression of the symptoms may result from direct typically constricted at the compost fungus from the roots, or from direct infection caused by splash of surface, this symptom being termed attack of the crown or stem base contaminated growing medium or wire-stem. tissues. plant material.

4a Rhizoctonia solani infection resulting in root rot in Calluna 4b Characteristic webbing (web blight) growing over the leaves of Calluna caused by Rhizoctonia solani Table 1 Table 2 Some common container-grown hardy ornamentals susceptible Some common container-grown to Phytophthora root, crown and basal stem rot hardy ornamentals susceptible to Rhizoctonia root, crown and basal Hardy nursery stock Pittosporum stem rot Abies Rhododendron Acer Ribes Hardy nursery stock Aesculus Rosemarinus Azalea Alnus Rubus Calluna Buxus Sambucus Ceanothus Calluna/Erica Senecio Cupressus Castanea Sorbus Erica Chamaecyparis Syringa Euonymus Cypressus Taxus Forsythia x Cupressocyparis leylandii Thuja Ilex Choisya Vaccinium Rhododendron Cordyline Viburnum Rose Escallonia Vinca Eucalyptus Herbaceous perennials Weigela Fagus Cheiranthus Fremontodendron Clematis Herbaceous perennials Gaultheria Hellebore Campanula Hebe Meconopsis Cheiranthus Juniperus Nymphaea Convolvulus Lavendula Saxifrage Dianthus Malus Euphorbia Oleaeria Hellebore Picea Lavendula Prunus Lupin Nepeta Saxifrage

Disease sources in the ground, in compost, in dust New infections are initiated when blown around the nursery, in aquatic resting spores are stimulated to and spread environments such as puddles and germinate by exudates released from surface drainage water, or within the roots of a potential host plant. Much of the propagation of hardy irrigation tanks or reservoirs, hence the Germination of a resting spore results ornamentals is by vegetative means. importance of good nursery hygiene. in release of myriads of spores which If this process takes place using infected plants or cuttings contaminated with Phytophthora, Pythium or Rhizoctonia, extensive disease spread may occur. Hence the importance of producing healthy stock plants.

Phytophthora and Pythium Phytophthora and Pythium are not seed-bourne but are often introduced to a nursery by infected cuttings. The means by which they survive and spread are very similar. Within infected tissues, spread is by fungal strands which can also cause new infections. Survival is by micro-scopic but very resilient resting structures produced within infected tissue. When these tissues decay, the resting structures can survive for long periods (months 5 Phytophthora root and stem rot in Chamaecyparis lawsoniana. Affected plants initially lose or possibly years) in a dormant state colour and wilt and later turn brown (above) are motile in films of moisture and growing season to the next. The disease spread on hardy perennials may be attracted by roots of fungus also produces structures in the UK. Spread usually occurs susceptible hosts. consisting of densely aggregated by growth of the fungus, over the Occasionally, when conditions are strands of the fungus (sclerotia). compost surface from an infected exceptionally wet, for example where These range in size from being cutting or plant to a healthy one plants outdoors are subjected to barely visible to as much as 6 mm nearby. This can happen very rapidly heavy rainfall, significant direct spread across and are variable in shape where cuttings are being rooted but to leaves and shoots may occur as and flattened in appearance. is much slower once plants are in a consequence of rain splash of They are longer lived than fungal individual containers and spaced out. contaminated material. This aerial hyphae, probably for a year or As with Phytophthora and Pythium spread occurs much more commonly more, but their precise role as a species, when conditions are with Phytophthora than with Pythium. means of perpetuation of the exceptionally wet significant direct pathogen in commercial nursery spread and damage to aerial plant Rhizoctonia stock production is not clear. parts may occur as a consequence Rhizoctonia is able to survive as Unlike many other fungi, spore of rain splash spreading material fungal strands in and on dead plant production by R. solani is not contaminated with the fungus. material and in the soil, from one considered to be significant for

Conditions Rhizoctonia susceptible to damaging attack, Information about conditions that hence any adverse cultural factor conducive to are particularly conducive to attack which slows or delays establishment disease by Rhizoctonia is not very precise. and growth will increase the risk of However, there is general agreement Rhizoctonia being a problem. Cuttings development that the fungus is more damaging are thus particularly vulnerable to when the growing medium is kept Rhizoctonia during rooting because Phytophthora and Pythium wetter than optimum and the aerial of the high humidity conditions The presence of a water film is environment is humid. Temperature necessary for rooting, which also essential for spore movement and can also markedly affect activity, encourage growth of the fungus. infection. Consequently, disease 20-30°C being the most favourable. development is almost invariably Plant tissues and stem bases while more rapid and more extensive where still young and soft are particularly the growing regime is wetter than that optimum for growth. Temperature also plays an impor- tant part in the speed of disease development. However, the ranges over which activity occurs, and those which are optimum for disease development, vary considerably between the different species of Phytophthora and Pythium. For example, some species of Pythium grow well at temperatures below 15°C whereas others grow best at 25–30°C. With Phytophthora, there is little or no germination of resting spores below 10°C and most species are much more active in the 20–30°C range. There is evidence that in addition to excess moisture, other adverse cultural factors (eg moisture stress, nutrient imbalance, excessively high conductivity, incorrect pH, or other factors resulting in root loss or a check to root growth) may encourage development of Phytophthora and Pythium. Therefore, the risk of signifi- cant damage is almost invariably greater where the root environment 6 Phytophthora root infection may progress into the stem base causing a red brown discolouration is less than ideal for growth. of the wood (left: Chamaecyparis; right: Fremontodendron). Control strategy propagation. Growers are best Nursery hygiene advised to obtain rooted plants There are many means whereby Use of healthy propagation material of Rubus species from reputable Phytophthora, Pythium and/or and maintenance of a very high specialist propagators. Rhizoctonia can survive on a nursery standard of hygiene is crucial for and rapidly cause problems. It is controlling Phytophthora, Pythium • Where possible, grow stock plants therefore important that a compre- and Rhizoctonia. For a strategy to be in free-draining growing media in hensive hygiene regime is developed wholly successful, these measures individual containers with generous and implemented. must be combined with good cultural spacing between plants. • Remove dead and dying plants management and rational and promptly. Thoroughly collect up and responsible use of fungicides. • Preferably locate the standing area dispose of plant debris and used under protection and ensure that it is growing media into a covered skip Provision of healthy propagation disinfected with Jet 5 prior to use or bin. material and is very well drained. • Carefully inspect cuttings arriving • When beds are empty, clean and on the nursery and reject any • Where possible, water stock plants disinfect the surface (eg with Jet 5). showing symptoms of root disease. individually, by trickle irrigation. Where there has been a significant root disease problem recently, • Select stock plants to take cuttings • Ensure that watering is well managed replace any ground-covering from which are vigorous, have so that the growing medium is material. strong,healthy-looking root systems never too wet nor too dry. and which are free from obvious or • Only use new or thoroughly cleaned suspected disease. • Where feasible, train plants to and sterilised pots and containers. discourage prostrate growth and Store them where they will not • Take cuttings from stock stimulate production of cutting become contaminated with dust, plants from sites where there has material higher up. water or debris. been no recent history of root disease problems. • Follow a comprehensive regime of • Regularly clean and disinfect the preventative fungicide treatments propagation area, including tools, • Because Rubus species are both for stock plants. equipment and benches. Before very susceptible to Phytophthora disinfecting, remove as much plant root rot, and are propagated by root • Promptly and carefully remove debris, dirt, spent media and dust cuttings, there is a high risk of suspected diseased plants and as possible. Rooting media should perpetuating the fungus during dispose of them safely. be sterile and not re-used.

• Regularly clean and disinfect all working areas and machinery linked to potting operations and the preparation of growing media.

• Disinfectants with activity against Pythium and Phytophthora include Jet 5 and sodium hypochlorite.

• Use proprietary growing media and substrates for use in own-mixes from a reliable source and store where they will not become contaminated.

• Ensure good bed drainage to minimise surface puddling and run-off.

• Check irrigation systems, especially overhead spray lines for leaks, drips or application irregularities.

• Maintain roadways to minimise surface puddling.

7 Branch dieback of Ribes caused by Phytophthora Irrigation • Preferably use mains or borehole water, or otherwise use water from a source where there is minimal risk of pathogen contamination.

• Check irrigation water at the point of application several times each year to ensure it is not contaminated by Pythium.

• Regularly clean and disinfect water storage tanks and irrigation lines (eg with sodium hypochlorite or Jet 5). Tanks should be covered to prevent contamination.

• Where there is a known higher risk of pathogen contamination (eg where water is collected and recycled), unless this can be minimised by other means, have a facility to treat it prior to use (eg UV-light, chlorine treatment or slow sand-filtration), particularly in propagation situations. A grower’s guide to slow sand filtration is available from the HDC. 8 Puddled roadways can allow spread of Pythium and Phytophthora on vehicle wheels, shoes and Cultural control during production by direct water-splash • Regularly monitor plant roots for the presence of disease.

• Manage watering so that the growing media is neither too wet nor too dry. This requirement can be much better met if production takes place under protection.

• Use well-managed trickle or sub-irrigation as much as possible eg drip-point irrigation, capillary sand-beds or ebb & flow, where appropriate, are ideal.

• Space plants as generously as the economics of production allow.

• Manage the aerial environment to minimise surface wetness and periods of high humidity.

• Use a growing medium which has the physical and nutritional properties eg texture, drainage, air- filled porosity, nutrient balance and level, which is most appropriate for the needs of the plant species being grown, and monitor nutrient levels to assess on-going needs. 9 Space plants well and use trickle or sub-irrigation where possible to minimise leaf wetness Use of fungicides • Determine frequency of fungicide Extension of Use (LTAEU) allow treatment based on duration of extrapolation of use from a label Successful, cost effective control of protection afforded by any previous approval or SOLA to use on an the major root and crown rot diseases treatment and the current level of ornamental crop at grower’s own risk. means: threat from the diseases eg more However, use must be in a similar • Use commercially economic frequent treatment may be required manner and rate as that specified on non-pesticide measures as much after a particularly wet spell, or the label or Specific Off-Label as possible. following damage to roots. instructions; be cautious as some treatments may prove to be • Where there is significant disease • Ensure that each fungicide applica- phytotoxic. Fungicides which are risk, use a preventative fungicide tion is applied thoroughly and at the effective against Phytophthora and treatment. Early fungicide treatment correct rate of use. Pythium are not usually effective before the disease is established, against Rhizoctonia, and vice versa. is important for effective control. Product selection There are a limited number of Fungicide application • Ensure fungicide products for fungicides with activity against Effective protection against Pythium and Phytophthora are Phytophthora, Pythium and Phytophthora and Pythium requires incorporated evenly into the Rhizoctonia (Tables 3 and 4). the fungicide to be thoroughly and compost or applied to the compost The range of fungicides permitted on evenly incorporated throughout the surface according to the label HNS and likely to be effective against compost. This can be achieved either recommendations. one or more of these diseases, is by thoroughly mixing etridiazole very much more extensive than that products or fosetyl aluminium directly • Alternate fungicides from different listed in these tables, particularly for with the growing media just prior groups to avoid the build up of production outdoors. This is because to use, or using these products or resistant pathogen strains. the Long Term Arrangements for propamocarb hydrochloride, applied

Table 3 Fungicide products with label approval for use on container grown hardy ornamentals and activity against Phytophthora and Pythium

Fungicide Active Product Use outdoors Level of disease control Comment group ingredient (O) Under protection (P) Phytophthora Pythium

Thiadiazole Etridiazole Standon O & P Label approval. Etridiazole 35 Do not use on Escallonia or Pyracantha spp.

Terrazole 35 O & P Label approval. WP Do not use on Escallonia or Pyracantha spp.

Phosphonate Fosetyl- Aliette 80 WG O & P ( ) Label approval for aluminium use outdoors; use IT Fosetyl-AL O & P ( ) under protection is by extrapolation Standon O & P ( ) from LTAEU and Fosetyl-AL 80 therefore at growers WG own risk.

Carbamate Propamocarb Filex O & P Label approval. hydrochloride Proplant O & P Label approval.

control Note: drench to the compost surface, according to the ( ) partial control All products should be incoporated throughly label recommendation. and evenly into the compost or applied as a Table 4 Fungicide products with label approval for use on container grown hardy ornamentals and activity against Rhizoctonia

Fungicide Active ingredient Product Use outdoors (O) Level of disease Comment group Under control protection (P)

Benzimidazole Carbendazim Cleancrop Curve O & P ( ) SOLA1213/2004. Apply (MBC) as a drench treatment.

Delsene 50 Flo O & P ( ) Permitted by extrapolation from SOLA 1004/2004. Apply as a drench treatment.

Dicarboximide Iprodione Rovral WP O & P ( ) Extrapolation under LTAEU from various crops. Use as a foliar spray; not approved as a growing medium drench treatment.

Cleancrop O & P ( ) As for Rovral WP. Gavotte WP

IT Iprodione O & P ( ) As for Rovral WP.

Ipromex 50% WP O & P ( ) As for Rovral WP.

Standon Iprodione O & P ( ) As for Rovral WP. 50 WP

Organo- Tolclofos-methyl Basilex O & P Label approval. Do not phosphorus use on heathers. Apply as a drench treatment. Use by hand-held equipment is not permitted. An organo- phosphorus pesticide.

Strobilurin (QoI) Azoxystrobin Amistar O & P Limited information on efficacy. Permitted by LTAEU from various SOLAs (eg 1687/02 on outdoor salad onions; 1533/02 on protected tomato). Use as foliar spray, needs to be high volume.

control • Some products recently widely used for root www.pesticides.gov.uk) or seek other ( ) partial control disease control (eg Aaterra WP, Fongarid, professional advice if in doubt about which Bavistin DF) are no longer produced, although fungicide products are permissible on HNS some stocks may still be available. or how to correctly use them.

• Always follow label recommendations, including • Important – Growers must always check the rate of use, and check the range of ornamentals current approval status of products listed in listed which can be safely treated. this factsheet before intended use as this could have changed since it was produced. • Contact the Pesticide Safety Directorate Information Service (01904 640500 or visit as a drench to the surface. Drench Fungicide resistance resistance have been very rare, and treatment is usually best started If certain fungicides are used inten- none confirmed in the UK. Products immediately after sticking or potting. sively, there is risk of selecting out based on iprodione (eg Rovral WP) Where there is continuing risk of strains of fungal pathogens that are and Carbendazim (eg Delsene 50 Flo) attack, additional fungicide appli- resistant.These fungicides then become may also give control when applied cation will be required. This interval much less effective or completely as a drench to the growing media. should be adjusted according to the ineffective.This has occurred with time of year. For example, few if any certain fungicides which have been Biocontrol and the use of disease applications may be needed during widely used in the past for controlling suppressive growing media the winter period in the cooler parts Phytophthora and Pythium on hardy There is continuing interest in the of the UK, especially where plants are ornamentals but which are no longer development of disease suppressive stood outdoors. available (eg Pythium species resistant growing media. These may utilise For protection against Rhizoctonia, to furalaxyl, the active ingredient in biocontrol agents, such as species the fungicide may only need incor- Fongarid). The fungicides currently of the fungus Trichoderma, or poration into a shallow surface zone approved and widely used for several suppressive substances produced (1–2 cm deep). Plants are most years for controlling these pathogens in bark or during composting. There vulnerable to stem and root rot attack (etridiazole, fosetyl aluminium and are also products being marketed, when young, and become progres- propamocarb hydrochloride) have not eg compost teas, which claim to sively less so as they mature. Early to date given rise to a resistance enhance the vigour and health of treatments are very important. Where problem. plant growth, and thereby strengthen there is a continuing risk of attack, Currently, effective fungicidal control the plant’s resistance to disease additional applications will be required. of Rhizoctonia on hardy nursery stock attack. Although there is limited Basilex, Cleancrop Curve and Delsene in the UK is very largely reliant on information on the reliability of such 50 Flo are permitted as drench tolclofos-methyl. Fortunately, even methods under nursery conditions treatments while iprodione products though this fungicide has been used and in comparison with fungicides, are permitted as a foliar spray. widely for many years, instances of some results have been promising.

Acknowledgements

We are grateful to Andrew Hewson, ADAS, for helpful comments. Photos 4a and 4b courtesy of Mark Mc Quilken, SAC.

Further information: A full copy of the final Whilst publications issued under the auspices © 2004 Horticultural Development Council. report for HDC project PC 97a is available from of the HDC are prepared from the best available No part of this publication may be reproduced in the HDC office (01732 848383). information, neither the authors or the HDC can any form or by any means without prior permis- accept any responsibility for inaccuracy or sion of the Horticultural Development Council. liability for loss, damage or injury from the appli- November 2004 cation of any concept or procedure discussed. Design and production: HDR Visual Communication