Solving the Case of Crop Disorders: The Role of Patterns in Diagnosis
Driving along your fields, what catches the eye? You expect to see a strong crop stand, but don't. How can you figure out the problem? With some basic plant knowledge and key observations, you can accurately diagnose and manage your plant problem.
Plant problems can be caused by biotic organisms or abiotic disorders. Biotic organisms that damage plants include insects, mites, animals, and pathogens. Abiotic disorders are caused by nonliving factors such as environmental extremes, nutrient imbalances, physical injury, and phytotoxicity from chemical applications. Distinguishing between abiotic disorders and biotic organisms using patterns is a key piece of the diagnostic puzzle. Below is a discussion of how abiotic disorder patterns can help to ID problems and guide management.
Five patterns of field crop symptomology:
1. Symptoms present but no signs
If you are looking at a stunted, discolored, dying, or dead crop but can find no visible signs of a pest or pathogen, then an abiotic disorder may be suspected. Many abiotic disorders can cause symptoms without visible signs, such as chlorosis caused by nutrient deficiency, wilting caused by water stress, reduced vigor caused by planting depth or sidewall compaction issues, and stunted plants caused by carryover herbicide injury.
On the contrary, biotic organisms are typically diagnosed as the cause of the plant problem when you see their physical presence (insects or pests themselves or the remnants they leave behind such as frass (Figure 1)) and corresponding damage on the plant. Confidence in a biotic diagnosis is easy when the organism can be clearly spotted but be careful as many organisms are microscopic and not easily visible with the naked eye. Additionally, a biotic organism may be responsible but no longer visible due to environmental conditions or life cycle. Further exploration of pattern development can allude to the cause.
Figure 1. The symptom on soybean is feeding damage, the sign is the presence of a slug. Photo: Tosh Mazzone
2. Symptoms observed across different species
Abiotic disorders differ from biotic organisms in that they often affect many plant species at a time, or multiple ages of different plants in a field. For example, when scouting barley at Spring green-up (Figure 2) chlorotic leaves on the crop and neighboring weeds suggested a lack of plant available nitrogen. This pattern commonly occurs with environmental extremes such as water stress from drought or waterlogged soils.
When a biotic organism such as fungal pathogen colonizes a plant, they only do so on a susceptible host, and many neighboring unrelated plants (such as weeds) remain unaffected. That is because many pathogens and pests have limited host ranges. If everything is affected, it is more likely that an environmental factor is involved. Â Â
Figure 2. Symptom pattern is chlorosis on both barley crop and weeds, which suggests an abiotic disorder such as nitrogen deficiency. Photo: Tosh Mazzone
3. Uniform damage
Damage from abiotic disorders is typically uniform along a pass, in a clearly defined area, or an entire field. For example, barley can exhibit chlorosis on leaves after the coldest days of winter, from inherent lack of winter hardiness (Figure 3) while neighboring wheat or cereal rye appear a healthy green. Herbicide injury caused by overlap or tank contamination will often cause symptoms that appear along a pass in the width of the spray boom or in a line at the edge of a pass. Furthermore, fertilizer applications that vary in rate can also cause nutrient deficiency symptoms such as nitrogen stress (Figure 4) that follows a pattern approximating the width of the fertilizer spreader.
Figure 3. Symptom pattern is chlorosis on barley within a uniform area- a single planter pass. Photo: Tosh Mazzone
Figure 4. Symptom pattern is chlorosis at a right angle in corn field, with a clearly defined border. Photo: Tosh Mazzone
We can contrast this with biotic organisms that spread over time. Biotic organisms will have a point of origin in the field related to their biology and life cycle. An insect may originate from a field edge while a root rot pathogen typically originates in a low-lying, waterlogged section of a field. The area of origin can eventually become an epicenter of severely affected plants. Biotic organisms will advance throughout the field with a more random, irregular pattern than abiotic disorders.
4. No symptom progression
Looking more closely, once an abiotic injury has damaged a crop, it will not progress further throughout the plant. For example, hail damage in early summer can cause shothole damage of young soybeans in the field, but these shotholes will not enlarge and new leaves will remain unaffected. This pattern can also be observed with planter issues (Figure 5), wherein single plants, rows, or planter widths exhibit stunted or delayed maturity of crops.
Figure 5. Symptom pattern of stunted plants limited to a single row within a pass. This row is one growth stage behind the rest of the planter width and will remain behind the whole season. Photo: Tosh Mazzone
A biotic example contrasting this pattern is slug damage. Slug injury on seedlings will appear as ragged holes in leaves. After subsequent nights, more feeding injury will appear, even until plants are completely consumed. Biotic organisms will continue to damage plants over time according to their life cycle.
5. It happened overnight…or did it?
Too often we have heard that "it died overnight". Unfortunately, this answer is often the result of insufficient scouting and unfamiliarity with the crop. However, there are some abiotic disorders that do truly happen overnight. Acute abiotic disorders stem from a single inciting event. Once that event is over (such as with frost, hail, or wind) plants can regrow out of the damage. Abiotic disorders that are chronic, such as nutrient deficiencies, will not dissipate on their own, but damage may be minimized given the correct fertility at the correct time. Drought can have both acute and chronic effects. Acute drought injury causes temporary wilting (Figure 6) where plants can regrow once conditions improve. Damage caused by chronic drought is more difficult to overcome.
Figure 6. Symptom pattern where drought-stressed corn will resume regular growth given sufficient rainfall. Photo: Tosh Mazzone
Biotic organisms can be confused with chronic abiotic disorders because both stressors cause decline over time. Damage from pests and pathogens may ebb and flow with weather, site conditions, and crop susceptibility, making diagnosis even more challenging. Â Frequent scouting for signs along with detailed site history information can pave the way for an accurate diagnosis.
Abiotic and Pathogen Complexes
Often abiotic and biotic plant problems occur simultaneously, or one can predispose the plant to another such as opportunistic stalk rot pathogens causing disease on drought-stressed corn. These biotic/abiotic interactions are difficult to diagnose to a single answer and as such, management is complicated. In the case that no obvious symptoms, or altogether too many symptoms are present, it is time to ask for help with your crop problem.
Where to get help
Contact your county extension office for help and resources in diagnosing plant problems.
Expert diagnosticians are available at the Penn State Plant Disease Clinic. The clinic provides free clinical diagnosis of plant diseases for Pennsylvania residents.
Have an unidentified insect that you need help with? The Penn State Insect Identification Lab provides identification services with management recommendations.
Many abiotic issues start with the soil. A soil test can reveal issues that may be affecting your crops. Plant tissue analysis may be useful to identify hidden nutrient stress in crops. Soil and plant tissue testing with the Penn State Agricultural Analytical Services Lab can be helpful to contextualize plant problems and plan future management.
