Growth cracking is an external non-infectious physiological disorder of the potato plant in which the tuber splits while growing. Growth cracks generally start at the bud or apical end of the potato and can extend lengthwise. Growth cracks make fresh-market tubers unattractive. Severe growth cracks can even impact the quality of chip potatoes and affect processing. Growth cracks form due to fluctuating environmental conditions, such as uneven soil moisture, soil and air temperature, and rapid water uptake and tuber growth. Growth cracks increase when relatively poor growing conditions, such as prolonged moisture stress or high temperatures followed by excessive irrigation or rainfall. Injudicious use of nitrogen fertilizer is applied in excess or when the application of nitrogen fertilizer is not properly timed. There is also a link between low boron levels in the soil and increased intensity of growth cracks.
The lenticels, or natural pores, of the potato tubers are ordinarily inconspicuous slits on the tuber surface. These lenticels become enlarged when potato tubers are allowed to remain in wet soil after digging, or when freshly dug tubers are stored in a place where the air is very moist. When lenticels are enlarged, numerous scabs like openings appear on the tuber skin. The tissues around many of these small openings become raised and appear to be pushed out from below. Frequently this tissue becomes corky inappearance. The damage done to the tuber by enlargement of the lenticels is solely to its appearance.
Second growth, often called knobby tubers, is a condition produced in potatoes most frequently when dry weather prevails during midseason and is followed by a rainy period. Some varieties, particularly those that produce long tubers, are more subject to knobbiness than other varieties. The second growth occurs on the main or primary tuber at sites where eyes are located. These growths vary considerably in shape and size. No internal tuber symptoms or plant symptoms occur. In dry weather, tuber growth ceases. After a rainy period, growth begins again. When tuber growth resumes, it is not uniform. Proliferations, or knobs, occur at the site of one or more eyes. Sometimes affected tubers have pointed ends. This abnormal growth response may also occur as a result of irregular irrigation.
Sprout Tubers (Secondary Tuber Formation):
“Sprout tubers,” “secondary tuber formation,” or “potatoes with no tops” are common names for second growth. This irregularity occurs when the rest period is completed in the spring after the seed is planted or when conditions are unfavorable for normal vegetative growth. A poor field stand is the first sign of secondary tuber formation. Although seed pieces are firm, a small, marble-sized potato has grown directly from an eye. If sprouts form, they are short and they terminate in new tubers.
Internal heat necrosis can be described as tan to brown spots in the parenchyma tissues of harvested tubers. The spots first appear near the apical end of the tuber. This disorder also has been referred to as internal browning, physiological internal necrosis, internal brown fleck and chocolate spot. Symptoms generally are not observed on tubers or foliage, except some cultivars may express blackened eyes, tuber distortions and depressed skin or shrunken legions on the tuber surface.
Black heart is a physiological disorder caused by lack of oxygen, leading to dark, necrotic cavities. The symptoms are an irregular-shaped area that turns black to blue-black with a distinct border. This disorder can occur during tuber development, before harvesting or in storage. During tuber development and harvest, conditions that favor low-oxygen availability such as compacted soils can lead to the development of blackheart. A water film surrounding the tuber (oxygen diffuses slowly through water) and water-logged soils also will favor blackheart development. Potatoes in storage can develop blackheart when bins are closed, piles are deep with poor ventilation, tubers are harvested when they are muddy or an excess of soil is in the storage facility, or when a water film covers the potato.