7/17/25 - Hot next week; Wet conditions continue

• Weather History

  • On July 17, 1972, slow-moving morning thunderstorms dropped 5.5 inches of rain at Sioux City, setting the all-time daily rainfall record there (still standing). Other highlights: 4.63 inches at Sac City, 3.45 inches at Humboldt.

• Short-Term Weather Outlook

  • Cool air has returned after a recent frontal passage, but temperatures are forecast to increase again heading into next week.

  • There are chances for showers and thunderstorms through the weekend.

  • A “Ring of Fire” storm pattern will set up as a heat dome builds south over Missouri, leading to more storms along its northern edge (ridge riders/squall lines), likely north of Iowa, affecting the Upper Midwest (ND, SD, MN, WI).

• Medium-Range Weather Setup

  • The bullseye of the heat dome appears over Missouri July 22–26; during this time, Iowa will see a precipitation gradient, with most rain favored in northeast Iowa.

  • Precipitation forecast is variable, and storm placement will become clearer closer to each event.

  • Southeast Iowa (and previously drier northern counties) have recently received significant rainfall, shifting them from "have-nots" to "haves." Ottumwa remains in moderate drought ("D1") as a stubborn dry spot.

• Climate Outlook

  • 6–10 and 8–14 day forecasts show high probabilities for above-normal warmth (especially south), then continued wetness through late July.

  • The 3–4 week outlook through August 8 gives no clear signal on temperatures, with a slight lean toward dryness in southwestern Iowa.

  • Initial August outlooks show “equal chances” for temperature and precipitation over southern and central Iowa, a result of persistent widespread soil moisture which helps moderate heat.

• Atmospheric Impacts

  • Recent wetness tamps down temperature extremes due to moist soils and increased cloud cover.

  • The area’s high dew points indicate low atmospheric demand, reducing drought risk but potentially increasing plant disease issues.

• Past Week's Weather

  • Severe weather last Friday produced two weak tornadoes in southern and eastern Iowa, with no widespread damage.

  • Wildfire smoke from Canada arrived over the weekend after a strong cold front and wind shift to northwesterly flow.

  • Wet conditions continued across much of the state.

Specialty Crop Impacts

• Foliar Disease Risk

  • Prolonged wet conditions are increasing foliar disease risks in specialty crops. Preventive fungicide programs are recommended for conventional growers, especially on brassicas (cabbage, broccoli, cauliflower).

  • Summary of biopesticide efficacy for brassica diseases (per Meg McGrath, Cornell University):

    • For black rot: Oxidate worked in 1 of 1 trial, Regalia in 1 of 2, copper in 4 of 5, Double Nickel in 1 of 1.

    • For alternaria: copper is most effective.

    • Over-use of copper risks resistance and soil accumulation; organic certification requires soil copper testing before use.

  • For basil downy mildew: Only resistant varieties (notably the Prospera series) provide reliable control—fungicides are not effective.

  • For powdery mildew on pumpkins and squash: Use potassium bicarbonate products like Kaligreen or Milstop (these are safer than baking soda due to plant salt sensitivity).

• Disease and Cost Monitoring

  • Meg McGrath's cost-per-acre chart helps growers compare costs of biopesticides and fungicides

• Beet and Swiss Chard Disease

  • Cercospora leaf spot is appearing on beet and Swiss chard leaves, making chard unmarketable. Resistant varieties like Char Bell are advised for chard.

• Japanese Beetle Management

  • Listener asked about Milky Spore for Japanese beetle grub control. Dan cautions little evidence supports its effectiveness and prefers pest exclusion over soil treatments, since beetles migrate from other areas.

  • Pheromone traps may attract more beetles than they catch—sometimes best given to neighbors to ward beetles away from your crops.

• Colorado Potato Beetle Resistance

  • The second generation of Colorado potato beetle larvae has emerged. Rotate insecticide chemistries to avoid resistance (e.g., use Azera in the second generation if spinosad was used in the first).

  • Non-chemical methods, like knocking beetles into a soapy water bucket, remain effective—plants tolerate up to ~33% defoliation before yield loss.

Announcements and Events

• The Fruit and Vegetable Field Day at Iowa State University’s Horticulture Research Station will be held August 5th, 2:00–5:30 PM (free, with supper and registration link provided).

• The Plant Peddler "Educate the Educators" Day is July 31st in Cresco, with a feast and entertainment; the next day is Variety Day showcasing mature bedding plants (trade show only, not for sale).

• Practical Farmers of Iowa opened applications for the horticulture program coordinator position.

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