Here are some of the best free agronomy/advisory sites for learning crop protection basics and resistance management. I’ve focused on sources that are credible, educational, and practical.

1) University Extension Sites

These are often the best free resources for foundational learning.

• University of Nebraska–Lincoln Extension

  • Strong crop protection and pesticide stewardship materials
  • Good on weeds, insects, diseases, and resistance management

• Iowa State University Extension and Outreach

  • Excellent pest management guides and crop protection basics
  • Very practical for field crops

• Purdue Extension

  • Good pest identification, disease updates, and resistance topics
  • Often easy to read and well organized

• Penn State Extension

  • Strong integrated pest management and disease resources
  • Good introductory materials for beginners

• North Dakota / Kansas / Minnesota / Illinois Extension

  • All have strong crop-specific advisories and resistance-management content, especially for corn, soybean, wheat, and small grains

2) Crop Protection and Resistance-Focused Organizations

• ACCase / HRAC / WSSA resources for herbicide resistance

  • Good for understanding herbicide mode of action and resistance
  • Especially useful if you want weed resistance basics

• FRAC (Fungicide Resistance Action Committee)

  • Excellent free material on fungicide resistance management
  • Great for learning mode-of-action group rotation and anti-resistance strategies

• IRAC (Insecticide Resistance Action Committee)

  • Good free guidance on insecticide resistance
  • Helpful for insect control and rotation concepts

These are among the best sources for learning the “why” behind resistance management.

3) Government and Public Extension-Type Sites

• USDA and state agriculture department sites

  • Often provide pest alerts, scouting guides, and crop protection notes
  • Useful for local/regional context

• CABI Plantwise / PlantwisePlus

  • Very good for crop problem diagnosis and practical crop protection learning
  • More global in scope

4) Practical Crop Protection Learning Sites

• Crop Protection Network

  • Great for disease identification and management in field crops
  • Very useful for corn and soybean disease basics

• Bugwood / IPM image libraries

  • Helpful for pest and disease identification with photos

• AgFax / regional pest update newsletters

  • Good for current advisories, though not always “learning-first”

5) Pesticide and IPM Education Sites

• National Pesticide Information Center (NPIC)

  • Very good for pesticide safety basics, labels, and exposure concepts

• EPA IPM resources

  • Good overview of integrated pest management principles

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Best places to start if you want a learning path

If your goal is to learn crop protection from scratch, I’d start with:

1. A university extension site for your main crop
2. FRAC / IRAC / HRAC/WSSA for resistance basics
3. Crop Protection Network for disease management
4. NPIC for pesticide and safety basics

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Quick recommendation by topic

• Weed resistance: WSSA / HRAC + university extension weed guides
• Fungicide resistance: FRAC + Crop Protection Network
• Insecticide resistance: IRAC + extension insect management guides
• General IPM: Extension services + EPA IPM resources
• Pesticide safety: NPIC

If you want, I can also give you:

1. a ranked top 10 list with links, or
2. a beginner learning roadmap for crop protection and resistance management.