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Field Notes Talks Crop Disease Diagnosis Resources

By Angie Peltier and Brett Arenz

The following information was provided during a 2024 Strategic Farming: Field Notes session. Use your preferred podcasting platform or listen online to a podcast of this Field Notes session hosted by UMN Extension crops educator Liz Stahl.

Crop Disease

Plant disease is an “impairment of the normal state of a plant that interrupts or modifies its vital functions” (Britannica). There are both abiotic causes of plant disease (think nutrient deficiency, flooded fields, herbicide injury, drought) and biotic causes. Just like animals, plants can be infected by bacteria, fungi, nematodes and viruses. Plants can also be infected by organisms called oomycetes, viroids, phytoplasmas and parasitic plants. Three things need to be present in order for plant disease to occur: a susceptible host plant, a virulent pathogen and environmental conditions that favor the interaction of the host and pathogen.

For example, symptoms of Goss’s blight and wilt (caused by the bacterium Clavibacter michiganensis subspecies nebraskensis) are bound to appear in some Minnesota corn fields each growing season because: 1) no hybrids are completely resistant to the bacterium that causes Goss’s wilt, 2) the pathogen survives the winter in residue from previously infected corn crops and 3) the pathogen enters the plant through wounds caused by sandblasting, hail or violent weather, conditions that are bound to occur in many places in the state each summer.

Digital Crop Doc

During the COVID-19 pandemic in 2020, in an effort to avoid spreading the virus to the clientele we serve, UMN Extension implemented travel restrictions that required personnel in need of travel to seek permission ahead of time. As a result, many of the services that some Extension educators had routinely provided, such as field visits to assist with disease diagnosis, were significantly delayed.

That is when Digital Crop Doc (DCD) was born. With financial support from the Minnesota Soybean and Minnesota Corn Research & Promotion Councils, and the assistance of Phyllis Bongard, an educational content development and communications specialist with UMN Extension, the Digital Crop Doc webpage was brought online.

Anyone that grows corn, soybean, sugarbeet, oats, barley, wheat, rye or forages in Minnesota and encounters an unknown disease problem is welcome to make a submission using a form on the DCD website. Diagnosis through DCD is free of charge as it is considered a preliminary diagnosis. Some diseases such as those that are caused by more than one pathogen may not allow a visual diagnosis through DCD and so for those submissions, submitting plant samples to the UMN Plant Disease Clinic would be recommended.

By design, the DCD submission form is smartphone-friendly so one can make submissions from anywhere with or without an email account. However, a phone number is required both for submitters to answer additional questions important to arrive at a diagnosis and for UMN Extension to provide a visual diagnosis and associated management recommendations.

Crop Disease Diagnosis = context clues + game of 20 questions (at a minimum)

Disease diagnosis requires a combination of symptom observation, information regarding patterns of symptomology and field history and often some dialogue with the submitter. For example, if one observes leaf lesions on soybean plants, knowing whether the symptoms tended to be more prevalent toward the bottom of the plant (such as one can observe with Septoria brown spot) or top of the plant (such as one can observe with bacterial blight or Cercospora leaf blight) is important. Similarly, knowing whether one observes symptoms in patches that tend to congregate in lower spots of the field (such as can be observed with Phytophthora root and stem rot) or across the entire field (such as one would observe with herbicide injury) is important information that can assist in diagnosis.

Up to 10 photos can be included with each submission. Tips and tricks for capturing photos that can aid in diagnosis can be found on the DCD webpage.

There have been submissions that require additional consultation with the submitter. One submission that required this additional consultation originated from a soybean farmer near Cologne, MN in Carver County. This gentleman dug plants in his field and split stems and roots after this consultation to provide additional information that eventually resulted in a diagnosis. Come fall, this farmer even shared with the DCD team his soybean yields in the affected area of the crop, which appeared to be unaffected by the symptoms observed earlier in the growing season.

UMN Plant Disease Clinic

The UMN Plant Disease Clinic (PDC) is nearing its 70th anniversary since its establishment in 1956. Before its establishment, faculty members in the Department of Plant Pathology were unsustainably burdened by requests for assistance in diagnosing what was plaguing their ornamental or crop plants or trees. The Plant Disease Clinic is a fee-based diagnostic service offered by the UMN Department of Plant Pathology and Brett Arenz directs the efforts of this 3-person operation. The clinic is located just across from the Minnesota State Fair fairgrounds in Stakman Hall. For those looking to drop off rather than ship plant samples, the clinic website has very detailed instructions for how best to find them on campus.

The clinicians at the PDC receive approximately 2,000 plant samples for diagnosis each year. In a typical year, samples are from at least 150 different host species, from homeowner-type submissions such as trees, shrubs or summer annual ornamental plants, to the array of vegetable and fruit species grown either in home gardens or for fresh market or processing to agronomic crops like corn, soybean, canola, sugarbeet, small grains, hay and pasture crops.

In order to be self-sustaining, the PDC charges a fee for their diagnostic services. Fees begin at $50 for simple diagnoses that only require examination under a microscope. More complex diagnoses that require isolation of the pathogen in the lab or molecular diagnostic techniques can cost approximately $100 per sample. However, as many of the practices that can be implemented to manage plant disease can be quite costly, the PDC fee can be a small price to pay to receive an accurate diagnosis.

Recommendations for sending samples to the PDC

For those that live near St. Paul, the PDC will accept samples that are hand delivered. But luckily for the rest of us in this geographically vast state, the PDC will also accept samples that are shipped to the lab. The PDC has great information on their website about how best to collect a sample that will result in a diagnosis and how best to ensure that the sample arrives to the clinic in a condition that will allow an accurate diagnosis. A sample that spends days in a hot truck before arriving at the PDC is often more difficult to diagnose. 

In general, it is best to focus on collecting plant samples from those living plants that are experiencing the most severe symptoms. It is much easier to determine the cause of a plant’s symptoms when living plants are submitted to the PDC. This is because there are other microbes in the environment called saprophytes that can colonize dead plant tissue, complicating diagnosis. The transition zone between diseased and healthy plant tissues is typically the most useful for diagnostic purposes.

Source : umn.edu

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