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

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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
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