LEAK ('shell rot')

Pythium spp. (2 main species), fungus; no foliar disease

Several species of Pythium cause this rancid internal decay of potato tubers. The fungi is strictly soil borne, survives a long time and found in most soils but especially in wet areas where it overwinters in debris. Pythium attacks many crops and weeds so it can't be eliminated through crop rotation.

Pythium leak


Tubers get infected only from infested soil. It is primarily seen at harvest and during early storage. Infection commonly occurs at harvest through wounds or bruises during hot and/or wet harvest conditions. The disease decays tubers but is not transmitted between tubers in storage.

Early Symptoms:

Very moist, gray or brown lesions form around wounds or near stem end. Internal starch is breaking down thus cutting the tuber shows a grayish creamy inside which darkens to black upon exposure to air. [note - the blackened tissue looks like blackheart, a physiological disorder, but leak is watery.] A noticeable vinegar-like smell develops; for me this is a really distinguishing symptom.

Later Symptoms:

A liquidizing of the tuber's inside is very noticeable and cavities may form. The skin will remain intact, a papery shell, unless ruptured ("shell rot"). Also very noticeable is a strong stench much like rotting fish. No mold is usually seen unlike Fusarium rots. [note - The black-bordered creamy cavities are similar to bacterial soft rot (blackleg), but leak is not slimy and it has that smell. In many cases, leak and bacterial soft rot will occur together since both are favored by the same conditions.]

Control Practices:

Leak enters tubers during harvest only through wounds — cuts, scrapes, skinning, and shatter. Wounding is absolutely necessary for leak infection; leak is not seen in or on tubers before harvest. Entry of and tuber rot by leak is promoted when air temperature at harvest is 75F and above or when tuber pulp temperature is 65F and a big problem if it's 70F or higher. Infected tubers begin to rot in 36 hours at these temperatures. Tubers harvested from overly wet areas are more prone to leak. In storage, leak doesn't move much from tuber to tuber. But at 50F, leak-infected tubers are easily invaded through lesions by soft rot bacteria (Erwinia) which can easily spread in storage among tubers. Late stages of leak can resemble freeze damage.

Tubers in storage need to keep cool, keep dry and keep well-aerated.

Tips to Prevent Leak

  • Avoid harvesting swampy areas.
  • Allow tubers to mature, skins to set.
  • Avoid mechanical bruising during harvest.
  • Apply metalaxyl (Ridomil) during early bulking and/or in-furrow at planting.
  • Harvest when temperature is below 75oF.
  • Don't use a water flume to move tubers.
  • Cure tubers for 3 weeks at 45-50oF.
  • Cool potatoes rapidly to 40-45oF for storage.
  • Keep humidity low during cooling.
  • Force air over tubers continuously during cooling.
  • Low temperature and humidity will stop leak from growing and dry out infected tubers.