Management Strategies for Botrytis Blight & Stem Canker on Greenhouse Tomato

Biology & Disease Cycle of Grey Mould (Botrytis cinerea)

Grey mould (Botrytis cinerea) can be a serious problem on greenhouse tomatoes under cool and wet (high humidity) conditions. In British Columbia, the fungus can cause serious damage to greenhouse tomato in early spring and late fall. B. cinerea is a cosmopolitan fungus that has a wide host range, including tomato, pepper and lettuce. The fungus can infect almost all parts of a plant, including stem, leaf, petiole, peduncle and fruit. Stem infection through leaf-pruning scars and peduncles (fruit stem) often leads to stem canker, the most destructive stage of the disease causing substantial crop losses.

Grey mould thrives under cool, wet condition and often establishes on dying tissues. It produces masses of dry spores called conidia that are air-borne. Spores are readily dispersed by wind (air currents), over-head irrigation or sprays, tools (particularly pruning shears & knives), machinery and workers. An epidemic situation can happen from air-borne spores that can infect soft tissues, cut-wounds and blossoms when moisture is present.

Figure 1. Botrytis infection on tomato (var. 'Bizarr') stem, showing brownish-grey, dry lesion and girdling of the stem. Masses of grey colour Botrytis spores can be seen on the surface.

The most common and damaging infections take place through leaf-scars during de-leafing operation. Botrytis spores can remain dormant for 10 to 12 weeks within leaf scars made at pruning. Such spores can be triggered to germinate by low light, plant stress or shift in fruit load. Spores germinate and penetrate the plant surface within 5 to 8 hours on wet/moist plant surfaces at the optimum temperature of 15-20°C. A new infection can produce visible symptoms and masses of spores (conidia) within a few days, thus a multiple cycles of infection can be expected in a given growing season.

Botrytis can survive/overwinter as mycelia and/or sclerotia in the soil, on plant debris, and on perennial plants and weeds for several months or years.

Figure 2. Botrytis infection on tomato (var. 'Bizarr') stem and fruit peduncle. Infected tissues turn brownish-grey and masses of grey Botrytis spores can be seen on the surface.	Figure 3. Tomato (var. 'Bizarr') leaf infected with Botrytis showing brownish-grey discolouration and withering.

Management of Botrytis

PREVENTION (Steps to be taken before the onset of disease):

• Follow overall biosecurity procedures: Enforce strict biosecurity & phytosanitary requirements as appropriate for your greenhouse operation.
• Prevent initial infection and introduction of Botrytis into greenhouse: Enforce general year-around sanitation practices and thorough year-end clean up (thoroughly disinfect greenhouse physical structures, machinery and tools between crops).
• Grow resistant/tolerant varieties if any: Avoid growing tomato varieties that are highly susceptible to Botrytis. Choosing a resistant if not a tolerant variety will reduce disease severity and build up of spores during critical infection period.
• Adopt good cultural practices: Remember! Cool & wet conditions are ideal for Botrytis outbreaks. High humidity (>80%) is highly conducive for disease development. To avoid this, leave adequate spacing between plants, increase air circulation by removing lower leaves from overly shaded areas. Maintain adequate heat and ventilation, particularly in the nights where temperature can be expected to drop significantly. Monitor the moisture level in the greenhouse. Avoid over-head irrigation. Avoid spray operations in the late afternoons and on cloudy days, spray operation in the morning hours is highly recommended. Any water-based irrigation or spray operations must be done in the morning hours and on sunny days to minimize prolonged wetness on plant surfaces. Avoid puddling of water on the surfaces of greenhouse production sites.
• Scouting and early detection of Botrytis infection: Periodically and closely monitor for Botrytis disease symptoms, especially in the spring and fall seasons. Follow a strict and structured fungicide spray program (see below) and cultural practices to prevent the spread and severity of the disease.

ERADICATION / MANAGEMENT:

• Follow a scheduled fungicide spray program: Begin using fungicides at the very fist sign of symptoms. Expected results may not be achieved with any good fungicide program if the disease pressure is high. See below for a proposed fungicide-spray program.
• Follow strict sanitation and cultural practices as outlined under PREVENTION.

• Most importantly, eliminate or minimize the inoculum (spore) load & sources of inoculum in the greenhouse:

1. Remove heavily infected plants from the bay. When doing so, if possible, wrap the infected plant tissues with a wet paper towel/newspaper (use soap water or Chemprocide) to prevent spores from dislodging into the air.
2. Cover cull/trash piles of infected plants with a plastic sheet and take immediately to a far site for deep-burial or incineration. Consider the greenhouse location and the wind direction when choosing a plant refuse dump-site.
3. Treat the Botrytis lesions on stems at a very early stage – scrape off the epidermal layer of the tissue and immediately apply an appropriate fungicide paste (Ferbam, see below).
4. Severe lesions cannot be treated, as described in step (3), since they have already damaged the vascular system; for such lesions, cover the lesions with disinfectant-treated paper towels/newspaper to reduce spore dispersal.
5. Apply Botran at 7-day interval or as required to infected lower stems up to 60 cm from the ground.
6. Treat the tarped-flow periodically with an appropriate disinfect.

• De-leafing operation and worker sanitation practices: One of the primary sources of Botrytis infection is via leaf-cut wounds, which often result in severe stem canker. Pruning is recommended in the early afternoon since it will allow leaf-cut wounds to dry quickly. Treat pruning shears and knives with disinfectant after pruning each plant. Use a couple of pruners alternatively; this will give sufficient time to disinfect a pruner (dip pruning shears/knives in either 70% ethanol or 0.1-0.2% Chemprocide for a minimum of 2 min). The second factor is handling of infected plants and de-leafing of plants by the workers. Use hand-sanitizers as frequently as possible, particularly when de-leafing.

• Suggested Fungicide-spray program to minimize the impact of Botrytis on greenhouse tomato:

CATEGORY I. Greenhouse with a high Botrytis disease pressure

• Pristine (1 application)
• After 7-14 days, apply Decree (1 application)
• After 7-10 days, apply Botran, Rovral or Ferbam (if disease persists, repeat the application every 7 days).
• If the disease pressure is still high, apply 1 more application of Decree followed by repeated applications of Botran, Rovral or Ferbam

CATEGORY II. Greenhouse with a low Botrytis disease pressure

• Pristine (1 application)
• After 7-14 days, apply Botran, Rovral or Ferbam (If disease persists, repeat the application every 7 days).
• If the disease pressure increases, apply 1 application of Decree followed by repeated applications of Botran, Rovral or Ferbam

CATEGORY III. New crop in a greenhouse with a previous history of Botrytis

• Apply Botran, Rovral, Ferbam or Prestop preventatively. Repeat this application every 7-10 days (for Prestop, every 3-4 weeks) in early spring and late fall.
   
• If disease appears, apply 1 application of Decree followed by repeated applications of Botran, Rovral or Ferbam. If disease pressure increases, apply Pristine (1 application only, if not applied previously) or 1 more application of Decree followed by Botran, Rovral or Ferbam.

REMEMBER!

• Decree is restricted to 3 applications and Pristine to 1 application per year. Since Botrytis can be expected to cause damage in spring and fall use these two chemicals wisely.
• Use the biological fungicide Prestop preventatively, before the onset of disease & at very low disease pressure. It may not give expected results at high disease pressure.
• To prevent resistance development in the pathogen to a fungicide,
  • Never apply a fungicide below recommended rate
  • Use fungicide at the highest recommended rate
  • Do not apply the same fungicide repeatedly. Rotate fungicides from different chemical groups.

[To modify greenhouse conditions and minimize Botrytis incidence, please refer “Greenhouse Vegetable Production Guide” 1996/1997, pages 31-33]

Table 1. A summary of registered fungicides and label information (Please adhere to Product label instructions when using each chemical).

PHI - pre-harvest interval
REI - re-entry interval
NA – Information is not available

Prepared by:

Dr. Siva Sabaratnam,
Plant Pathologist,
Food Safety & Quality Branch,
Ministry of Agriculture & Lands

Revised: July 3, 2008