VG04012 Hydroponic lettuce – root rot

Hydroponics is a quick, clean and efficient production system for leafy lettuce. Growers are spread across Australia providing a fresh supply of lettuce to central and local markets.

Root rot diseases occur sporadically and hamper production efficiency. They cause major losses in hot weather and are an impediment to the expansion of the industry.

The purpose of this project was to develop effective and economic
management strategies for root diseases of hydroponic lettuce.

This report details research into casual pathogens, major environmental factors in order to develop effective and economic management strategies for root diseases of hydroponic lettuce.

Along with assessment of potential biocontrols the project identified greater levels of disease were associated with higher temperatures, moisture stress, poor crop and harm hygiene and infection with tomato spotted wilt virus.

Author : Len Tesoriero

VG04012 Effective management of root diseases in hydroponic lettuce - 2008
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Summary :

Root rot diseases cause seasonal, sporadic and sometimes entire crop losses in Australian hydroponic lettuce crops.

This project characterised the pathogens responsible, and determined their relative importance and distribution.

Two Oomycetes were mostly associated with diseased roots. Phytophthora cryptogea was the most aggressive pathogen confirmed by pathogenicity assays.

Pythium coloratum and closely related species were very common in roots, but only a few isolates were demonstrated to cause disease symptoms.

Both Pythium and Phytophthora were commonly isolated from symptomless root samples throughout the year emphasising the relationship between disease expression and certain plant stresses.

Seedlings (particularly those grown on the same site as the hydroponic lettuce production) were sometimes infected with pathogens suggesting one potentially significant means of entry into recirculating nutrient systems.

  • One highlight has been to demonstrate consistent disease suppression by a strain of the bacterium, Bacillus subtilis formulated as a microbial biocontrol product.

  • This product is being developed for registration as a bio-pesticide. Disinfection strategies were evaluated but have strong limitations.

  • Chemical disinfectants are toxic to roots at concentrations required to kill pathogens so their use should be restricted to non-crop use. Some were toxic to plant roots at even lower concentrations. They have an important role for farm and source water sanitation.

  • UV-light and sonication were effective disinfection strategies for recirculated nutrients but they are expensive and require infrastructure changes to farms.

  • Since higher nutrient temperatures were generally associated with greater disease, finding economical and practical ways to maintain them at lower temperatures remains a challenge.

  • Evaporative coolers and passive heat exchanger coils placed in nearby dams are being used commercially but they have limited effect under extended hot weather conditions.

  • Growers also use shading, plastic screens or overhead watering to reduce heat stress.

  • All integrated crop management strategies require growers to tailor those that are practical and suitable to inpidual production systems and geographic locations.

  • Some good disease management strategies are universal though. Poor farm and crop hygiene were strongly correlated with increased disease.

  • It is most important to start production with healthy and uninfected seedlings and diseased plants should be removed and disposed of, both timely and hygienically.

Recommendations :

  • Sustained and effective management of root rot diseases of hydroponic lettuce will require an ongoing commitment by growers to minimise the predisposing factors identified in this study.

  • In particular, it is recommended that growers adopt sound hygiene and sanitation practices to prevent pathogens establishing in their units and to reduce the risk of them spreading.

  • Using a few lettuce of the very susceptible cultivars identified in this project could be a useful way of monitoring for early disease expression in production units.

  • Use of larger seedling plug sizes were shown to result in larger plants in the presence of plant pathogens, however they need further evaluation under conditions of high disease pressure.

  • The use of microbial biocontrols should be encouraged but further studies are required to determine optimum use patterns.

  • Of the other potential biocontrols assessed in this project, Pseudomonas putida and Streptomyces lycius were shown to give intermediate control of root rots.

  • These and other potential biocontrols should be tested further and their compatibility assessed as mixed formulations.

  • Some overseas studies have demonstrated compatible combinations of microbial biocontrols that act in a synergistic way, thereby increasing their efficacy.

Acknowledgements :

Key Personnel:

  • Dr Leanne Forsyth, Plant Pathologist, EMAI, Menangle
  • Mr Jeremy Badgery-Parker, Extension Officer, HRI, Gosford
  • Mr Roger Carrus, Technical Officer, EMAI, Menangle
  • Mrs Fiona Lidbetter, Technical Officer, HRI, Gosford
  • Mr Josh Jarvis, Technical Officer, HRI, Gosford
  • Ms Aida Ghalayini, Technical Officer, EMAI, Menangle
  • Dr Mary Ann Terras, Technical Officer, EMAI, Menangle
  • Ms Brenda Gorrie, Technical Officer, EMAI, Menangle
  • Ms Teghan Crowe, Technical Assistant, EMAI, Menangle

The research team wish to thank :

  • Rijk Zwaan Seeds Australia and Leppington Speedy Seedlings for providing seeds or transplants.
  • Zadco for Quality Gro Pty Ltd funded some efficacy trials with Fulzyme PlusTM and other microbial biocontrols.
  • Other companies (listed in the body of the report) provided their respective disinfectants, disinfecting equipment or other microbial biocontrols.
  • Collaboration is acknowledged with Dr Khalaf Alhussaen who completed his doctorate study on Pythium and Phytophthora Root Rots of hydroponic lettuce with The University of Technology, Sydney in 2006.
  • Technical support is acknowledged from the NSW DPI Plant Health Diagnostic Service, Menangle.
  • Finally, we would like to thank hydroponic lettuce growers in NSW, Queensland, South Australia and Victoria who provided access to their farms and provided plant samples for disease surveys.

This project has been facilitated by the NSW State Government, Horticulture Australia Limited (HAL)
in partnership with AUSVEG through the National Vegetable Research and Development Levy.

The Australian Government provides matched funding for all HAL’s R&D activities.

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