Root pathogens: Prevention and management in nursery production 

By Emma De Landre, Plant Protection Officer, New South Wales/Northern West Victoria 

Plant root pathogens can cause significant stock losses in nursery production. Limiting the spread of these diseases contributes to reduced waste and increased profit within the nursery industry, including your business. 

Beyond the farm gate, plants infected with root rot organisms can spread disease into urban landscapes, food production systems and the natural environment. Click on the following links for detailed information on common root rot pathogens including Phytophthora, Pythium, Phytopythium, Rhizoctonia and Cylindrocladium. Management can be challenging as many of these diseases have a wide host range and symptoms are not always present. Treatment may reduce symptoms and limit the spread of diseases such as Phytophthora however there is no cure, so prevention is the only effective method of control. This article will outline various management strategies, processes and resources that are available to assist growers in the prevention and management of root rot.  

Prevention 

Root pathogens spread via the movement of fungal, or oomycete, spores. These spores are not visible to the human eye and may be present in nursery stock, growing media, soil, surface water, re-used pots or soil transported by the movement of people, machinery, or equipment. To limit their spread implement nursery best management practice (NBMP) processes that will reduce the risk of disease. Detailed information is available in the Nursery Industry Accreditation Scheme Australia (NIASA) BMP Manual and BioSecure HACCP Manual on the Australian Plant Production Standard (APPS) website. Consider implementing the following strategies to manage stock imports, water sources, irrigation, growing media, re-use of containers, machinery, equipment, hygiene, and site management. 

Inspect stock purchased externally for signs of disease, pests, and weeds. When receiving stock, select a sample, and inspect the roots for any obvious symptoms of disease such as discolouration, an absence of ‘fresh’ roots or rotting. Healthy new roots are generally fleshy and white in colour. Reject plants that do not have a sufficient root system to sustain above ground growth. Plants can appear healthy above the surface of the container and still be infected with root rot. If it is not practical to inspect stock as it comes in, keep plants in a quarantine area and inspect stock before moving plants to the growing area. Consider the source of imported stock, purchasing from a NIASA accredited nursery reduces the risk of disease. Additionally, using starter stock propagated in a sterile environment using tissue culture techniques reduces the risk of plant pathogens.  

Disinfest surface water sources before using the water for irrigation. Water disinfestation treatments, used according to best management practice, kill plant pathogens. Methods include chemical treatment, microfiltration, and UV radiation. The choice of disinfestation systems depends on several factors including flow rates, pipe sizes, water quality (pH and EC), pump capacity and the distance from the pump to the irrigation outlet. For this reason, growers should consult an irrigation designer. Good filtration reduces turbidity and solids that host plant pathogens. When scheduling irrigation, avoid overwatering as this creates conditions for root diseases to thrive due to plant stress or water logging and encourages common plant pests such as fungus gnats. 

Growing media should be free of plant pathogens, so to ensure this use a NIASA accredited growing media supplier. Growers using non-NIASA accredited suppliers should ask for results of pathogen testing or alternatively pathology test each batch on site before use. Growing media manufactured on site must be composted using approved processes and comprehensive quality controls. Fungicides can be added to growing media as a preventative particularly during periods of prolonged wet weather. Follow these instructions to bait and test growing media for root pathogens using a rapid diagnostic test kit, Baiting Growing Media and Soil for Phytophthora and Pythium sp. 

Wash and disinfest re-used pots, containers or trays using steam or a sterilising solution. Production nurseries commonly use plastic fruit bins, emptied 1000L containers or tubs for soaking containers/pots which are raised or moved as required on pallets. Steam sterilisers are commercially available as portable units or fixed systems. View the following link for information on treatment times and temperatures. 

Avoid transporting soil from one production area to another on vehicles, front end loaders or earthmoving machinery. Limit vehicle access in production areas, access should be for staff to conduct day to day maintenance and despatch activities. For further guidance view the following video - BioSecure HACCP Vehicle Inspection Procedure. Sterilise tools and equipment used for propagation or pruning with disinfectant solution between each batch. 

Design outdoor production areas in such a way that surface water drains away from plants. Use surface screenings (gravel), as a barrier between the plant container and the soil surface or raise plants up onto benches (e.g. metal, plastic, etc). Splashback of surface water or soil onto containers increases the risk of spreading plant disease. Production nurseries designing new production areas can find useful recommendations in the NIASA BMP Guidelines. For businesses that are retrofitting growing areas, follow the same advice and consider dividing the production nursery into sections, according to risk, and tackle an area at a time. The initial outlay is likely to reduce throw outs and ongoing costs over time. Clean and disinfest empty growing areas or benches between batches. Foot baths or vehicle dips at entrances to production areas can be effective providing they are maintained. See this link for more information on nursery hygiene. Xxxxxxxxxxxxx

Identification 

Root rot pathogens may include Phytophthora, Pythium, Phytopythium, Cylindrocladium (Calonectria) or other less common root pathogens such as Rhizoctonia, Chalara, Fusarium, Aphanomyces, Sclerotinia and Sclerotium. Refer to the following link for detailed information on root rot pathogens and follow the steps below: 

• Refer to images of root rot diseases contained in the Pest ID Tool and the pests, diseases, and weeds section of the APPS website. 

• Inspect plant roots, often by the time above ground symptoms are present the damage to the root system has already occurred. Healthy, fresh roots often look white and ‘fleshy.’ Absence of fresh roots, brown or yellow discolouration can be a sign of root rot. Sometimes rotting will be visible and usually appears dark in colour. 

• Look for above ground symptoms which may include yellow foliage, brown or dead foliage and wilting despite adequate irrigation. Root pathogens attack the roots which reduces nutrient and water uptake. Plants infected with root rot pathogens may look healthy above the ground or growing media surface. Symptoms may not be present at all, particularly in the initial stages of infection. 

• Use field testing kits as a monitoring tool to identify problem areas or plants. Be aware that a false positive is extremely rare, however a false negative is common. Follow up positive results with a laboratory test as required. 

Professional laboratory services test growing media, water, or plant samples for root pathogens. Laboratories provide advice on correct sampling techniques. Please refer to the chart below for laboratories that offer root pathogen testing. Australian production nurseries may receive up to six free tests per year from Grow Help Australia under a current Nursery Levy and Hort Innovation funded project (‘NY20000 Resourcing, supporting, and assessing biosecurity in nursery production’).  

Table 1. List of Laboratories that Provide Testing for Root Pathogens 

Control 

If test results have confirmed that stock contains root pathogens, implement controls: 

• Isolate infected stock to reduce the risk of other plants becoming infected. 

• Identify the source of infection if possible. Check that drainage is adequate and monitor plants and weeds in surrounding areas for symptoms. 

• New stock imported recently that tests positive for root pathogens may be the source. Contact the supplier to advise. 

• Dispose of dead or infected stock. 

• Clean and sanitise the growing area with a disinfectant or product registered for control of root pathogens. Check registrations at the Australian Pesticides and Veterinary Medicines Authority (APVMA) or use a minor use permit if required.  

Note: Using fungicides on infected nursery stock will limit the spread and reproduction of root rot organisms. However, plants showing symptoms will not recover. Infected nursery stock should not be despatched as movement may transport disease to another location. Fungicides are often used for treatment of root rot in the urban landscapes. 

Management 

Management options may include development of site-specific procedures to prevent and control diseases in the production nursery, this can be brief and provide advice on internal processes. Train staff to recognise symptoms of root pathogens as they are the ‘eyes on the ground’ and can monitor for disease symptoms when working in growing areas or as part of a structured monitoring program. Implementing the nursery best management practice guidelines will reduce the risk of plant pathogens in the nursery. The philosophy of continuous improvement supports positive change, consistent small improvements over time contribute to significant long-term gains. View the following webinar delivered by Dr Andrew Manners, Senior Entomologist with the Queensland Department of Agriculture and Fisheries for further advice: Managing Root Rot Pathogens in Production Nurseries. 

For on-site assistance and guidance on implementing best management practice please contact the relevant Plant Protection Officer in your state or territory and organise an appointment: 

• Queensland/Northern NSW: Barry Naylor, barry.naylor@greenlifeindustry.com.au   

• New South Wales/Northern West Victoria: Emma De Landre, emma.delandre@greenlifeindustry.com.au   

• Victoria: Kimberley Thomas, kimberley.thomas@greenlifeindustry.com.au   

• Western Australia/Northern Territory: Steve Blyth, steve.blyth@greenlifeindustry.com.au   

• Tasmania and South Australia: Celeste Cook, celeste.cook@greenlifeindustry.com.au  

The ‘National biosecurity and sustainable plant production program’ (NY20001) project is funded by Hort Innovation using nursery research and development levy and funds from the Australian Government.