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We’ve talked about black scurf and sweet potato scurf. Today, we’re learning about silver scurf. Silver scurf may sound like the latest Marvel comic hero, unfortunately, it’s a fungal disease of potatoes. For the most part, silver scurf is a cosmetic problem, but infection can lead to tubers rotting in storage, and they don’t look as pretty. Well, pretty for a potato.  Silver scurf scabs on potato (A.J. Gevens, UW Extension) CC BY-SA 4.0 Potatoes infected with Helminthosporium solani can be shrunken, shriveled, and lighter in color. This disease causes spud dehydration and scabby bits. You may also see black spots, which are tiny fungal factories. Other conditions that result in scabby potatoes include common scab, potato scab, and powdery scab. Scabby leaves often indicate Eriophyid mite feeding or bacterial spot, and scabby apples are a warning to look before you bite, because it may be a sign of codling moth larvae feeding. Apples are also subject to apple blotch and apple scab, while cucurbits can look less than their best due to belly rot and cucurbit scab. Until 1977, Thiabendazole (TBZ) fungicide was used against silver scurf. But as is often the case, the fungi developed a resistance. Other fungicides have been tried, but cultural controls provide a more sustainable method of reducing the chance of disease. Silver scurf occurs most often when temperatures are between 59°F and 90°F (15°C and 32°C), so pretty much all summer. Tubers left in the ground too long can also increase the likelihood of silver scurf. Crop rotation and early harvest are good ways to avoid this problem. Silver scurf is not to be confused with the lovely legume, silvery pea-scurf (Pediomelum argophyllum).  Silvery pea scurf (Matt Lavin) CC BY-SA 2.0 Fusarium yellows is a disease of the cabbage family.
Closely related to Fusarium wilt, Fusarium yellows sometimes goes by that name. More commonly, it is simply called yellows. This disease is caused by Fusarium oxysporum f. sp. Conglutinans fungi. It occurs most often in crops grown during warm weather. The disease is at its worst when temperatures are between 80°F and 90°F (27°C and 32°C). The pathogen generally enters damaged rootlets at transplanting time, so be gentle with those babies. Symptoms of Fusarium yellows are easy to overlook at first. Infected plants become a paler version of their healthier selves. They develop a yellowish tint, rather than vibrant green. As the disease progresses, that discoloration becomes more obvious on one side of the plant. The leaves and stems on that side start to curve. Lower leaves, midribs, and petioles eventually wilt and die. Symptoms then move to the upper portion of the plant. If you look at a cross-section of an infected leaf or stem, you can see that the sap has turned dark brown, much like black rot, which turns sap black. The other difference is that symptoms appear in the upper portions of the plant first when black rot is the culprit. Infected plants should be removed and thrown in the trash. These fungi can live in the soil for many years. The best preventive methods include crop rotation, removing plant debris at the end of the growing season, and growing your brassicas during cooler weather. |
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