Pest and disease management - Phytophthora root rots

Treatments to defend against these pathogens should be used alongside good hygiene practice.

Phytophthora root rot - image: Dove Associates
Phytophthora root rot - image: Dove Associates

Phytophthora pathogens threaten ornamental horticulture and the wider landscape. Phytophthora spp. have been a major cause of root rots in both nursery stock and in bedding and pot plant production.

They belong to the oomycetes, which are closely related to the true fungi. They produce free-swimming zoospores as well as tough oospores for long-term survival, and infection depends on the presence of water.

In protected ornamentals, Phytophthora spp. attack the roots of young seedlings or cuttings, causing damping-off. In outdoor nursery stock, landscape plantings and woodland, they cause root and collar rots. With most Phytophthora spp. the disease is confined to the lower stem and root system, with some spreading in the aerial parts.

Although healthy cuttings can be taken from above the limits of infection, it is not good nursery practice. It should never be attempted from plants infected with notifiable species, including P. ramorum and P. kernoviae. Phytophthora spp. can also directly infect aerial parts if splashed.

On the nursery, there is a choice of fungicides to treat growing media to protect against Phytophthora root rot, but these must be used with good hygiene practices including the use of disinfectants, biopesticides and soil sterilisation on outdoor seedbeds to help minimise infection risk. Do not use fungicides to control Phytophthora root rots where they could mask notifiable Phytophthora diseases.

How to recognise it
There are no specific characteristics of the presence of these pathogens. Wilting, stunting or root damage may be caused by poor growing conditions or other pathogens. But, because of the ease with which Phytophthora spp. can spread, any symptoms that could indicate an outbreak should be investigated. Initially, a field test kit can be used. The species must be confirmed by further laboratory tests. Staining of inner bark or wood, caused by some Phytophthora species on woody plants, may help diagnosis.

Phytophthora spp. produce motile zoospores but they can only swim about 4cm, so spread depends on them being passively carried in moving water (including irrigation) or through the movement of infected plants, substrate or contaminated tools and equipment. Spores may also be transported by insects.

A moist or waterlogged root zone and warm soil or growing media are necessary for infection but symptoms may not appear until later — for example, when subsequent drought contributes to the plant’s inability to take up water. Temperature can play a key part in development. There is very little germination of the resting spores below 10°C. They are most active at 20-30°C.

On pot plants and bedding plants, Phytophthora spp. cause root and stem base rots, leading to wilting and plant collapse. On some plants, purpling or other leaf discolouration may be the first indication of root problems.

On nursery stock or older plants in the landscape, symptoms include wilting, yellowing or browning and retention of dried foliage, darkening of young roots, reduced root mass and, depending on the Phytophthora species, staining of inner bark or wood just above ground level.

Symptoms may be triggered by drought, the disease having been present and spreading for some time. They may spread through a crop — for example, from one end of a bed or from close to an irrigation nozzle — or from an individual infected plant.

Treatment: biological control

  • Compost teas or growing media ingredients, such as composted bark and composted green waste (to the PAS 100 standard), can produce a root zone ecology where Phytophthora spp. and other pathogens are less able to infect roots.
  • Products such as Prestop* (Gliocladium catenulatum strain J1446), Revive, Serenade ASO* (Bacillus subtilis strain QST713). Trianum G or Trianum P (Trichoderma harzianum) can be used in integrated disease management or prevention strategies.
  • Some nutrient products such as potassium phosphite may improve plants’ resistance to root rots.

Treatment: cultural control

  • Inspect newly delivered plants or plugs and monitor closely for weeks.
  • Avoid physical damage at transplanting and potting.
  • Do not pot plants too deeply.
  • Keep growing area clean of plant debris.
  • Ensure growing media or soils are adequately drained.
  • Avoid standing plants in water-soaked areas on beds.
  • Protect irrigation reservoirs or tanks and growing media stocks from contamination.
  • Use sand filters or sterilisation to clean irrigation water, especially if recycling.
  • Use disinfectants to clean capillary mats, reusable trays and so on between crops.
  • Only take cuttings from healthy stock — never from branches touching the ground.

Treatment: chemical control

Active ingredient Bacillus subtilis strain QST713

  • Formulation Serenade ASO* (Various) 
  • Action(s) Protectant biofungicide. Compatible with biological controls.

Active ingredient Dazomet

  • HRAC code Z
  • Formulation Basamid (Certis)
  • Action(s) Granular soil fumigant.

Active ingredient Fenamidone + fosetyl-aluminium

  • FRAC code 11 + 33
  • Formulations Fenomenal* (Bayer)
  • Action(s) Systemic fungicide. Not compatible with some biological controls.

Active ingredient Fosetyl-aluminium

  • FRAC code 33
  • Formulation Plant Trust (Everris)
  • Action(s) Controlled-release fertiliser prill with fungicide. Compost incorporated.

Active ingredients Fosetyl-aluminium + propamocarb hydrochloride

  • FRAC code 33 + 28
  • Formulation Previcur Energy* (Bayer)
  • Action(s) Systemic and contact fungicide. Not compatible with some biological controls.

Active ingredient Gliocladium catenulatum strain J1446

  • Formulation Prestop* (Various)
  • Action(s) Protectant biofungicide. Compatible with biological controls.

Active ingredient Metalaxyl-M

  • FRAC code 4
  • Formulation Subdue (Syngenta)
  • Action(s) Systemic fungicide with protective and curative action. Compatible with Trianum P.

Active ingredient Metam-sodium

  • HRAC code Z
  • Formulations Various including Discovery (United Phosphorus)
  • Action(s) Partial sterilant for glasshouses, nurseries and outdoor soils.

Active ingredient Propamocarb hydrochloride

  • FRAC code 28
  • Formulations Proplant* (Arysta)
  • Action(s) Translocated protectant fungicide. Compatible with biological controls.

Fully updated by Dove Associates

Use plant protection products safely. Always read the label and product information before use.
* EAMU required for use in ornamental plant production outdoors and/or under protection.
Dove Associates shall in no event be liable for the loss or damage to any crops or biological control agents caused by the use of products mentioned.

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