A is for Alternaria

Hearing a lot more about Alternaria diseases these days? Alternaria is the genus of two important foliar diseases in potatoes. The first is early blight, caused by the species Alternaria solani. The second is brown spot, caused by Alternaria alternata and other closely related species, which together are known as the A. alternata species complex.

Early blight and brown spot are considered endemic in Canada – they likely exist in almost every potato field at some level. All Alternaria spp. associated with these diseases overwinter on crop residues, alternate hosts, and tubers. Their spores spread by wind and rain, so their ever-present nature is unsurprising. A recent Syngenta disease survey completed in 2020 and 2021 showed that the proportion of early blight versus brown spot varies by region and year.

Alternaria diseases are known to wreak havoc on yield by limiting leaf growth (meaning less leaf area) and causing early maturity (via dead leaves). Both outcomes mean leaves have less chance to convert the sun's energy into the dry matter needed for tuber bulking.

Looking for Alternaria: Where, When, and What

Infections favour warm, wet conditions from dew, rain, or irrigation. These conditions often occur at flowering but can arise earlier or later depending on the year. The best practice is to start scouting before rows close. 

Infections start inside the canopy, so drive-by scouting is inadvisable if you want an early indication of crop health. Early blight symptoms are typically more severe when potatoes are under mid-summer stresses (e.g. nutrient deficiencies or infection by other diseases) and during alternating wet and dry periods. Brown spot is typically more prevalent in warmer seasons, and though it can occur at any time, it typically develops later in summer after rows close. Tuber infection is also a risk with this disease.

Typical symptoms of early blight and brown spot

	Earliest observed symptoms	Symptoms observed after infection is well established
Both	Pin-head sized dark spots on leaves	Larger, expanded spots; symptoms may eventually spread to stems and tubers
Early blight	Spots most often begin on the oldest leaves	Leaves with many spots with concentric rings and a target board appearance, sometimes with a yellow halo
Brown spot	Spots most often begin in the middle of the canopy	Spots with concentric rings that join to form large dark brown to black patches, often on leaf edges; leaf drop may occur

What spot is this?

Left: Picture of potato leaf with brown spots with concentric rings. By the descriptions above, it is hard to visually tell if this is brown spot or early blight.

Early blight and brown spot are so hard to distinguish even experts need to confirm diagnosis with more than a visual assessment. It is not unusual for a plant to be infected by both pathogens. Scouting for lesions on lower leaves can be challenging as they may already be yellowing due to shading by the upper canopy. If you need to know if it’s one disease or the other, a laboratory diagnosis is recommended, although they are often managed in a similar way with similar products.

Remember, early blight and brown spot are just two of many potato diseases that cause leaf spots, including Botrytis grey mould, Verticillium wilt, and black dot, not to mention abiotic stresses – so consider the big picture when diagnosing a crop.
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Ways to fight early blight and brown spot

Looking for the best way to fight early blight and brown spot? Use an integrated approach to manage Alternaria diseases in potatoes.

1. Implement a crop rotation of at least 2 to 3 years before planting potatoes or another host like tomatoes or eggplant in the same field again.
2. Manage plant stress including abiotic stress like drought and/or nutrient deficiencies.
3. Apply fungicides preventatively.

Product (FRAC Group)	Application timing and resistance management notes
(Group 3+11)	Best used just before row closure for black dot activity, with early blight and brown spot activity from difenoconazole (Group 3). There is widespread resistance to Group 11 fungicides in potato growing regions in Canada.
(Group M)	An important mixing and/or rotational tool throughout the season for single site mode of action fungicides when fighting Alternaria diseases.
(Group 3+7)	Use Group 7 fungicides prudently to delay resistance. Follow FRAC guidelines for the total number of applications in a program and do not apply more than twice consecutively when used in mixture.
(Group 3+7)

  

Curious about these or other Syngenta potato products?

Check out our Potato Hub for additional resources, product details, and application information. 


For more information, visit Syngenta.ca, contact our Customer Interaction Centre at 1-87-SYNGENTA (1-877-964-3682).
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