Stephen J. Toth, Jr., editor
Volume 19, Number 12, July 2, 2004

Fruit and Vegetables

From: Frank J. Louws, Extension Plant Pathologist

Strawberry plug plants are commonly used in the strawberry industry with multiple benefits. Plug plants establish with less overhead watering and management after field setting. Plant size and quality is generally higher and some growers prefer to plant plug plants as compared to bare root plants. Important diseases associated with strawberry plants include anthracnose, bacterial leaf spot, Botrytis crown rot, viruses and phytoplasma diseases, Rhizoctonia or Pythium root rot, Phytophthora crown rot, and powdery mildew. In the past 2 or more years we have seen a higher level of anthracnose and Phytophthora associated with plug plants and these problems typically are associated with contaminated tips. Contaminated plants or poor management during the plug production phase can result in substantial losses to plant quality and ultimately fruit production. Observations based on experience also suggests plug plants have higher mite populations than bare root transplants. Apparently, greenhouse conditions favor parasitic mites or limit populations of beneficial mites.

August and early September is the time for rooting tips to produce plug plants for field transplanting. Disease management tips to consider are:

1. Use tips from a reputable source. Tips can be infected with angular leaf spot, anthracnose or Phytophthora without showing symptoms. Although there are no guarantees, a reputable source of plants is a first wise step. Regretfully, there is no reliable way to sample tips and assure disease is not present. Nursery suppliers who grow plants according to strict certification standards and allow field inspections by independent parties may have reduced risk of problems.

2. Carefully cull tips and discard all diseased and questionable tips. If anthracnose is suspected, send samples to the clinic for a rapid diagnosis.

3. Sort tips by size and plant accordingly. Small tips rooted in the same tray as large tips results in excess shading of the small tips and provide ideal conditions for Botrytis rot and powdery mildew.

4. Avoid excess fertility and misting. Excess moisture and fertility favor disease.

5. Ensure adequate ventilation and air circulation. Ventilation and use of horizontal-air-flow fans are needed to limit heat build-up and excessively long periods of leaf wetness (as the misting requirements decrease). Based on field observations, outdoor plug systems appear to have less disease pressure than systems in greenhouses.

6. Be vigilant about sanitation, sanitation and sanitation. Field soil carried into the greenhouse on boots, equipment or plant materials may introduce pathogens such as Rhizoctonia and Pythium fungi that cause crown and root rot. Sanitation also includes removal of dead and dying plants in a timely manner and when practical. Maintain a weed free or well-mowed area around plug houses to limit disease problems from weeds or other crop plants.

7. Do not produce strawberry plug plants in any location where there is likely to be water originating from ornamental nursery plants. Ornamental plants harbor the pathogen Phytophthora cactorum and this pathogen causes a serious crown rot of strawberries. Never use surface water to water strawberry plants if the surface water drains areas of ornamental plant production. If practical, have trays above soil surface so water cannot move from one tray to the next.

8. The use of fungicides is a cloudy issue. Some production facilities produce plug plants successfully without fungicide applications. Others find fungicide use provides benefits in plant quality. Still others receive recommendations that are not legal or will not be effective against the main diseases of concern. According to the EPA Registration Division, if a label bears general directions for use on a crop without restrictions, such as, for outdoor use only, for field use only, or not for use in greenhouses, the EPA Registration Division would most likely determine that the use of that product in a greenhouse on the specified crop would be consistent with the label. It appears plugs produced outdoors would be exempt. Captan should not be used during the early rooting phase since it inhibits rooting. In our field trials where Captan was drenched (i.e., not sprayed on foliage) over plug trays just prior to field setting, plants remained stunted all year.

9. Specific Recommendations: These recommendations are not based on research results in plug production facilities but appear to be consistent with the label and experience. For fungicide rates, take the per acre rate, add this to 100 gallons and spray for adequate (not over soaking) coverage. Thus, if you need 10 gallons of water to cover the area, use 1/10 the per acre rate on the label.

   Phytophthora: Keep Sweet Charlie plugs separate from others. This variety seems to be very susceptible or planting stock from Canada and California seems frequently infected. Growers may consider using a phosphite-based product (e.g., Prophyt, Aliette, Phostrol, etc.) 2 to 3 weeks after setting. Complement this with field application of Ridomil Gold in fields with a history of Phytophthora. (see our previous recommendations on the following website: http://www.smallfruits.org/SRSFC_News/Phytophthora0404.pdf).

   Botrytis: Apply Switch or Elevate + Captan (e.g., CapteEvate) on a weekly schedule beginning 2 weeks after plug setting. E.g., week 2 = Switch; week 3 = CaptEvate; week 4 = Captan.

   Anthracnose: USE certified plants or plants from a reputable source. The Captan sprays for botrytis will suppress anthracnose. Outdoor plugs appear to get less disease than greenhouse plugs. Quadris, Cabrio and Pristine labels state "Do not use for disease control in food crops grown in greenhouses" or "Not for greenhouse or transplant production". If used on transplants, resistance may develop rapidly and then we will have a serious problem. If hot spots develop, cull these spots and surrounding trays. If the disease is widespread, seek an alternative source of plants. Initiate spring fungicide programs in problem fields (these have worked very well). See our previous detailed recommendations on the World Wide Web at: http://ipm.ncsu.edu/current_ipm/palert43.html.

Late blight has been identified in western North Carolina on tomatoes, and it continues to be very wet and cloudy with ideal conditions for late blight development. This is exceptionally early and growers are advised to adjust their fungicide spray schedules.

In Table 1, we have outlined the standard recommendations for 2004 in which a mancozeb or chlorothalonil product is recommended every other spray. Table 2 contains the application rates. This information is posted on the web at: http://www.ces.ncsu.edu/fletcher/programs/plantpath/Foliartext.htm.

With late blight pressure, growers are advised to include mancozeb weekly until harvest and then chlorothalonil weekly once harvest begins. Chlorothalonil offers superior protection against the fruit blight phase. Thus, if late blight is in the field, it may be wise to switch to chlorothalonil earlier. The strobilurins offer some protection against late blight but are not as good as weekly applications of the protectant fungicides like mancozeb and chlorothalonil. The strobilurins every 14 days are important for early blight control. Thus, growers should tank mix mancozeb or chlorothalonil with a strobilurin every 14 days and use it alone (or with copper) during the off week.

In fields where infection has already occurred, mefenoxam provides excellent systemic and eradicant activity against sensitive late blight strains. We are currently testing isolates for sensitivity but results will not be available until Friday or next Tuesday. In the last few years, the first infections have been caused by sensitive strains. Therefore, advise growers to make either one drip application of a mefenoxam 2E formulation (Ultra Flourish) or 4E formulation (Ridomil Gold) or one or two foliar applications 14-days apart of mefenoxam + chlorothalonil (Flouronil, Ridomil Gold/Bravo).

Growers who have a dense canopy and experience wet cool weather may want to include Endura earlier in their spray program. This has superior gray mold (Botrytis) control. Gray mold usually is a concern when wet weather persists, when the canopy is dense and when first harvest starts.

Finally, we have been receiving a number of questions about Tanos. Tanos has good early blight and late blight activity and has been shown to have some activity against bacteria. (Tanos is considered a Group 11 fungicide, just like Quadris and Cabrio and thus cannot be used as a rotation partner to the strobilurns. It also must be tank mixed with a contact fungicide such as mancozeb or chlorothalonil.) In trials at Fletcher North Carolina, the regular use of mancozeb or chlorothalonil has proven the most effective for late blight control. Likewise, our research with Actigard has demonstrated our program is the most effective for bacterial speck control. Tanos would probably not add much more control of speck. Thus, Tanos has not been incorporated into our recommendations yet.

Table 1. Suggested weekly spray schedule and products (x) for foliar disease control in fresh-market tomato production in North Carolina (JUNE 30, 2004: Add mancozeb or chlorothalonil weekly and tank mix with strobilurin).

Table 2. Amount of product per 100 gallons, assuming a maximum of 100 gallons spray per acre at full plant growth.

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Last modified on July 6, 2004 by Stephen J. Toth, Jr.