Systemic fungicides are important management tools because they are capable of moving within a plant to parts not easily reached directly by contact fungicide sprays applied with a conventional sprayer. Most systemic fungicides have a single site of action; unfortunately, many strains of fungi have developed with mutations at the target site that make the organism resistant (insensitive or tolerant) to the fungicide. Control failure can occur when resistant strains are common. If resistance management strategies are implemented before resistant strains become common, the effective life of the chemical can be extended. Most contact fungicides are not at risk for development of resistance because they have multiple sites of action.
Among cucurbit diseases, resistance is a concern especially for powdery mildew and gummy stem blight/black rot, and potentially for downy mildew. For the powdery mildew fungus, resistance to triadimefon (an active ingredient in Bayleton and Reach) and/or to benomyl (Benlate) has been found in several strains in New York and elsewhere in the United States. Strains resistant to benomyl are most likely resistant to other fungicides in this chemical group (e.g., Topsin M). More than one application of Bayleton or Benlate/Topsin M may not be effective, based on the relative frequency of strains resistant to these two fungicides prior to treatment in 1993-1995. Strains of the gummy stem blight fungus resistant to both benomyl and thiophanate-methyl (Topsin M) have been identified recently in New York and elsewhere in the eastern United States. Strains of the downy mildew fungus resistant to metalaxyl (Ridomil) have been found in Israel. For this reason, Ridomil is always applied in the U.S. for foliar usage as a mixture with protectant fungicides (i.e., Ridomil/Bravo, Ridomil MZ58 [mancozeb] and Ridomil + Copper).
The following resistance management strategies should be used.
1. Apply systemic fungicides only when FIRST needed. Scout weekly for symptoms to ensure applications are made at the critical time, which is the start of disease development.
2. NEVER apply systemic fungicides ALONE. Use systemic fungicides in combination with a contact fungicide (chlorothalonil [Bravo, or other chlorothalonil labeled formulation]).
3. Alternate between systemic fungicides with different active ingredients when possible.
4. Maximize spray coverage on leaf undersides if possible.
5. Limit the number of applications of systemic fungicides by using contact fungicides at less critical times.
The recommended fungicide resistance spray program for 1996 applied on a 14-day interval is:
1. Benlate or Topsin M + a chlorothalonil product at the highest labeled rates FOR BOTH after the powdery mildew threshold is reached (PM sporulation found on one leaf out of 50 examined). 14 DAY INTERVAL (WEEK 1)
2. Reach fungicide or the combination of Bayleton + chlorothalonil at the highest labeled rates FOR BOTH. 14 DAY INTERVAL (WEEK 3)
3. Chlorothalonil alone (WEEK 5), and continue regular applications until two to four weeks before the end of harvest, if practical. Add Benlate or Topsin M if gummy stem blight occurs or is of concern on the farm.
If downy mildew occurs, apply a Ridomil combination (14 day INTERVAL) or Aliette + maneb (7-14 DAY INTERVAL). This disease is more likely to occur on Long Island and the lower Hudson Valley.
Note: Most spray programs evaluated in research trials at Ithaca and Riverhead have been on a 7-day interval and have included chlorothalonil as the contact fungicide. A longer interval is recommended here because it is anticipated that it will be effective and that growers are more likely to use a 14-day interval rather than spraying on a weekly basis. However, if a 7-day interval is used, then chlorothalonil alone should be applied between the week one and the week three.
Reprinted from Capitol Vegetable News July 1996