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.
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.
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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