Plant tissue testing (Part 1) — Best sampling practices

By Greg LaBarge

Plant tissue testing is a valuable nutrient management tool. We often use it to identify if off-colored plants have a nutrient deficiency. Another place tissue testing fits into management is to evaluate our fertility program. This use can help identify any hidden deficiencies that could limit yield. Combining soil testing, which predicts the soil’s available nutrients and fertilizer needs, with tissue testing, which measures plant uptake, is a robust data set to make nutrient decisions. Tissue testing also fills a gap in nutrient management, where soil testing is unreliable for determining sulfur and micronutrient needs. If you use plant tissue testing, use these best sampling practices.

Collect the appropriate growth stage and plant part to compare to established sufficiency standards. To know if your test result is “good” or “bad,” you must compare your result to a standard that includes the yield response at the end of the season. Nutrient sufficiency ranges for most crops are available for samples collected at the mid-vegetative and early reproductive stages. Figure 1 demonstrates the importance of proper staging to time the sampling. Notice the variation in nutrient content when collecting leaves earlier or later than the target R1 corn stage. A sample collected at the wrong time can lead to the wrong conclusion when comparing results to established standard sufficiency ranges.

Figure 1. Variation in the nutrient content of corn ear leaf sampled before and after R1 growth stage. Source: CORN Newsletter, Issue 20-2018.

Nutrient sufficiency ranges in the Tri-State Fertilizer Recommendations Bulletin 974 are available for corn and soybeans in the V5 and R1 stages for corn and soybeans. For V5 samples, collect the entire above-ground (1 inch above soil surface) portion of the plant of 20-30 plants. For R1 samples, collect 15 to 20 ear leaves. For alfalfa, collect the top six inches before initial flowering.

Select plants from representative areas of the field. Avoid plants affected by insects, disease, or adverse weather conditions such as too much or too little water.

When the goal is to diagnose a nutrient deficiency, collecting samples from the affected area and comparing them with a nearby good area is a helpful comparison. In problem areas, target moderately affected plants collected from the transitional area of bad to good rather than the center of severely impacted plants. The most severely affected plants will have several nutrients out of balance, making it challenging to isolate the problem nutrient.

When collecting plants with symptoms, take pictures of the individual leaves, plants, and field areas. These may be useful if you have similar symptoms in other fields in future years.

Collecting soil samples when you are collecting tissue samples can be helpful. Pulling a tissue test sample and a soil probe at the plant just sampled provides valuable information, especially when diagnosing plants with symptoms. A soil sample may not be required when collecting tissue tests in established soil sample zones.  

One year of tissue testing is insufficient to evaluate your nutrient management program. Consider collecting 4-5 years of plant tissue analysis. Weather conditions, planting dates, and other crop production factors vary yearly and may impact your results.

If you have not included tissue testing in your nutrient management program, 2024 is an excellent year to start. Collecting samples in high and low-productivity areas or fields within your operation would be the place to begin. Next month, I will share some resources for interpreting your tissue test results.

Understanding tar spot development

By Stephanie Karhoff

Tar spot has emerged as a major foliar disease of corn in Ohio since its first confirmed diagnosis in 2018. It was identified in 2015 in Illinois and Indiana and has now been confirmed in 19 states and Ontario, Canada ( Tar spot can cause significant yield losses, primarily due to reduced ear weight, poor kernel fill, and, in severe cases, lodging. It can also reduce silage corn feed quality. However, disease prevalence and severity vary from year to year.

So, what influences tar spot development and its impact on your farm? In short, it boils down to three components — hybrid susceptibility, environmental conditions, and pathogen presence. These three factors comprise what is referred to as the ‘disease triangle’, and the interaction between host plant, pathogen, and environment is required for disease to occur. Let’s take a closer look at each of these three sides of the ‘disease triangle’ and how they influence tar spot development.

Host— Currently, no commercially available hybrids are completely resistant to tar spot, and susceptibility ranges from 1-50% of leaf area infected (Telenko et al., 2019). When selecting hybrids, consult with your seed dealer and review university corn performance trial data to avoid highly susceptible hybrids. Hybrid screening and breeding efforts are limited since tar spot is a relatively new disease, they will improve as tar spot becomes more established and controlled environment assays are implemented. It is important to note that a favorable disease environment can still overcome strong hybrid resistance.

Environment— Tar spot severity and timing of infection is driven by environmental conditions. Recent research based on small plot field trials and commercial fields in eight different states, including Ohio, showed monthly temperatures between 64 and 73 degrees Fahrenheit, relative humidity above 75%, and short periods of leaf wetness during early infection increased disease severity (Webster et al., 2023). Weather dictates regional differences in tar spot severity and influences when infection occurs. In general, tar spot is more problematic in years where infection occurs earlier in the growing season.

Pathogen— Tar spot is caused by the fungus Phyllachora maydis which produces small, raised black structures or fruiting bodies called stromata. It is endemic to Latin America and requires a living host to grow and reproduce. The pathogen can overwinter in the Midwest in infested residue and under the right conditions, will release spores that can spread by wind or rain (Figure 1). Spores can travel long distances (at least 4,100 feet), so even fields without a history of tar spot should be scouted. Infection can occur anytime during the growing season, and after 12- 15 days, new stromata are formed and can produce additional spores in a conducive environment.

Figure 2. Tar spot disease cycle. Source: Iowa State University Integrated Pest Management.

Overall, your hybrid, weather, and inoculum levels will determine the yield impact of tar spot this year. As the growing season progresses, follow the CORN Newsletter and use the tar spot forecasting mobile application Tarspotter (available at to guide scouting efforts and fungicide decisions.

Stephanie Karhoff, OSU Extension Field Specialist, or 567-376-4019

Greg LaBarge, OSU Extension Field Specialist, or 740-956-5047

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