Phytophthora erythroseptica, fungus; no foliar disease
Closely related to Pythium is the genus Phytophthora which includes the species causing pink rot, P. erythroseptica, and is related to late blight, P. infestans. Pink rot fungi live in most soil and survive long periods without any host. Unlike leak, pink rot can be detected in mature plants prior to harvest. It causes a late season wilt starting from the base and moves up the vine. Tubers can be dug up by hand and checked for pink rot before harvest.
Tubers get infected only from infested soil. Like leak, it commonly appears at harvest and early storage. Pink rot will spread from tuber to tuber in storage if tubers are wet. Infection occurs usually through the stolon and enters tubers through the stem end pre-harvest. However, reports have shown that pink rot can enter tubers also through wounds and swollen lenticels and eyes, and have a leak-like phase. As with leak, pink rot-infected tissue can easily be invaded by soft rot, Erwinia carotovora and tubers often rot in two weeks. Pink rot may move in storage from tuber to tuber although this is not fully confirmed. Infection of 5% of tubers in storage is considered unmanageable.
Pink rot may cause a wilt at the end of the growing season. The wilt starts from infecting the base of the stem and working up causing leaf yellowing, drying and loss. Aerial tubers may appear. However, pink rot is considered more of a tuber problem than a wilt.
The tuber surface shows dark lesions delineated from healthy tissue by a blackish band. Tubers tend to be flaccid and may give off a small amount of liquid when squeezed. The key symptom, which also gives the disease its name, is a pink coloring inside the tuber that develops in 10-20 minutes after cutting. The discolored area is not mushy or slimy, and is not well delineated as with leak or bacterial soft rot. In time, the pink will change to brown and then black. A slight pungent odor similar to formaldehyde may occur. Cavities do not develop as with leak.
Pink rot develops in poorly-drained soils and field areas, or under excessive irrigation/rainfall. Often plants growing next to wheel tracks or wherever there is stagnant water. It's most severe in wet soils that are at 68-86oF. Pink rot can survive on the roots of small grains.
As with leak and bacterial soft rot, pink rot develops when harvest conditions are warm (> 70 F) and wet. Avoid over-irrigating during senescence near the end of the season. Application of metalaxyl in the field at planting or when tubers are at "marble" and "golfball" sizes have shown to be effective.
Tips to Prevent Pink Rot
- Avoid excessive watering late in season.
- Delay harvest of swampy areas until checked for pink rot.
- Harvest when temperature is below 75oF.
- Avoid mechanical bruising during harvest.
- Apply metalaxyl (Ridomil) during early bulking.
- Cure tubers at 45-50oF.
- Cool tubers rapidly to 40-45oF after curing.
- Use continuous forced and adequate ventilation through pile.
- Leak (Pythium leak, shell rot) = Pythium spp.
- Pink Rot = Phytophthora erythroseptica
- Soft Rot (bacterial soft rot, blackleg) = Erwinia carotovora
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