Diseases affectingpa pa can develop in the field, during transport, in the warehouse, in the market or in our own home. Theydo not usually pose any risk to people or animals, but produce substantial losses in the harvest. Let’s get to know them.
Main potato diseases
- Potato diseases caused by fungi
- Mildew or late blight
- Pink rot
- Alternaria or early blight
- Powdery or roña scabies
- Warty scabies
- Silver scabies
- Potato diseases caused by bacteria
- Bacterial wilt
- Bacterial necrosis
Most of the fungi that attack the potato live in the soil or can be transported by wind, rain, soil stuck in the tools from other farms, insects, irrigation water and in the seed itself where they have spent a period of dormancy. They penetrate the plant through natural openings, such as lenticels, through wounds or through direct infection. Some mushroom families thrive in cold, wet climates and others in hot and dry climates.
- Mildew or late blight. Caused by the fungus Phythopthora infestan Bary is one of the most important diseases d the potato crop, causing serious economic losses worldwide. The infection begins in times of mild temperatures combined with high humidity and cloudy days and once transmission begins, it is favored by high relative humidity and high temperatures. The symptomatology in the aerial part begins with light green spots on the edges of the leaflet of the upper leaves progressing towards the petiole and the stem and becoming dark brown necrosing the tissues; sometimes attacks on the tubers occur without having appeared hardly any signs in the aerial part, and vice versa, severe infections in the aerial part that do not affect the tubers. In the tuber there is a dry rot in the reddish-brown surface area that advances progressively towards the interior of the tuber. Contaminated tubers are the main source of infection. As for the fight, we must use certified seed, sow the tubers to a medium or deep depth and use systemic fungicides such as Captan, Mancozeb, Folpet, Cimoxanyl, Fosetil Al, Maneb, Copper oxychloride and the combinations between them. Early detection of the disease is critical.
Symptomatology of mildew in potato leaf and tubers
- Pink rot Caused by the fungus Phytophthora erythroseptica Pethybridge is a disease present in Spain, but little known. The symptomatology in the plant is chlorosis, wilting and brown or black lesions at the base of the stem, roots and stolons, which can be confused with those that occur in the blackfoot of bacterial etiology. In the tubers appears a uniform rot with the area of advance delimited by a dark line visible through the skin. Lenticels take on a dark brown or black color. By cutting an affected tuber and exposing it to air, the internal tissues change from cream to pinkish or salmon color and subsequently blacken. It is a disease of practically underground development and the development of the disease is maximum in humid soils and with temperatures between 25 and 30ºC. As for the measures to be taken to fight against sickness is to avoid all kinds of waterlogging, eliminate sick potatoes after harvesting and store dangerous batches at a temperature below 10ºC; long rotations with non-susceptible crops and planting varieties that are not very sensitive to the disease since there are no resistant varieties.
- Alternaria or early blight. Disease caused by the fungus Alternaria solani Sorauer and the symptomatology is seen in the older leaves as circular spots with concentric rings that darken as they progress, producing the death of the leaf. It is not normal for it to attack the tubers, but if it is the case, dark sunken spots appear on the skin and the pulp below has a corky appearance; young tubers or tubers with wounds are prone to attack. The fungus winters in the remains of the previous crop and plants with virosis and lacking nitrogen are more prone to attack by the fungus. It needs free water on the leaves, so periods of dew or rain followed by temperatures around 15ºC are conducive to the fungus. To avoid alternaria problems, fungicides such as Mancozeb or Maneb should be used (mildew treatments are also effective against alternaria) and let the tubers mature well before harvesting them to avoid wounds through which the pathogen can penetrate. It does not attack the early potato. Among the recommended fungicides are Captan, Cimoxanyl, Difenoconazole, Folpet, Mancozeb, Maneb, Metalaxyl, Copper Oxychloride, Zineb and their combinations.
Symptoms of leaf and tuber alternaria
- Powdery or roña scabies Caused by the underground Spongospora fungus, the infection occurs through the lenticels, wounds or by the buds and occurs during the period of growth of the tubers; the pathogen lives in the soil. The symptoms appear as brown-purple pustules under the skin that when growing break the skin of the tuber appearing a kind of white wart. Preventive measures for the control of this disease are to use seed free of the disease, not to sow on contaminated land, to carry out wide crop rotations, to sow in porous and well-drained soils and to use resistant varieties.
- Scabies wart Sychytrium endobiocticum is the pathogen and the disease is characterized by the appearance of warts on the entire surface of the plant except at the root. The first thing that is appreciated are the appearance of warts at the base of the green or chestnut stems, on the stolons and tubers; sometimes they also appear on the leaves and inflorescences. Warts on tubers are soft and more or less spherical. There is no means of fighting the disease and the pathogen can remain active on a plot for up to 20 years. The disease must be combated through quarantines.
Warty scabies in potato
- Silver scabies Helminthosporium solani causes in the tubers a silvery shine that is observed especially when the tuber is washed or has wet skin. The pathogen penetrates through the lenticels and skin when the tuber is planted and conditions are high humidity. If the attack is strong, the skin of the tuber is destroyed, not serving the potato for consumption or seed. Disinfected seed should be used and extensive rotations should be carried out.
- Rhizoctonia Rhizoctonia solani Khun is a pathogen present in all plots where potatoes are grown and the conditions that favor the development of the disease are high humidity, low soil temperatures with a neutral or slightly acidic pH and soil with good fertility. Also known as potato pox,they look like sclerotia in the dark brown tubers which can be detached with the nail without leaving a mark practically on the tuber; under the skin there is no damage. Sclerotia differ from the remains of earth in that they are not washed. The most important damage occurs in spring and shortly after planting when the pathogen attacks the underground shoots causing nascence failures in the plantation. The use of disinfected seed does not always take effect in very infected plots so a practice to avoid the disease is to sow superficially and with pre-sprouted seed to prevent the shoots from spending a lot of time in the ground.
Rhizoctonia in potato
- Gangrene Poma exigua is the causative agent of the disease that affects the tubers during storage. In the wounds, lenticels or in the eyes of the tubercles appear dark depressions that elongate to form what are known as “thumb prints”, which are irregularly shaped lesions and well-defined edges. The inside of the tuber produces a dry rot
- Fusarium Several species of the genus Fusarium can cause problems in the potato, but mainly it is Fusarium solani and Fusarium oxisporum that are the most common. Also known as dry rot or warehouse rot, symptoms begin with a chlorosis of the old leaves followed by wilting and falling. The tissues of the stem turn brown from the bottom up. The disease is favored by temperatures between 15 and 25ºC and high humidity. In the infected tubers appear small brown necrosis that evolve into sunken spots with wrinkled skin, sometimes in the form of concentric rings; in the end the tubers remain dry and mummified. The inner fabrics take on a spongy appearance with brown colorations, well-delimited edges and cavity formation. When the skin on its surface breaks, the white mycelium of the fungus appears with its white or reddish fructifications. To avoid the appearance of the disease it is recommended to plant healthy seed, discarding the heavily contaminated lots, harvest when the skin is well formed and avoid all kinds of blows during collection and storage. In the case of having wounds we will favor the healing of these, properly handling the humidity and temperature.
Symptoms of Fusarium on skin and pulp
- Verticilosis The causative agent is Verticillium dahliae Kleb and is an organism that lives in the soil. Symptoms are wilting due to involvement of the xylem ducts. The initial symptoms are the yellowing of the leaves starting with the lower ones, following a flaccidity and general wilting of the aerial part. If we cut the stems we will appreciate a brown coloration of the xylem vessels, especially in the neck area; a characteristic is that the dead stems remain erect. In the tubers, internal necrosis is also seen in the vascular bundles of the stolon insertion area. It is a soil disease and the optimal temperatures for the development of the disease are 22-25ºC (temperatures above 30ºC can inhibit the manifestation of symptoms). The disease is favored by excess water, nitrogen and lack of potassium. The different varieties of potato have different response to verticilosis, and the most resistant varieties must be used in the plots with problems. Rotations with cereals or legumes should also be monitored and host weeds should be controlled.
Typical symptomatology of Fusariosis in potato
Bacteria are organisms that thrive in hot and humid conditions and usually penetrate tissues through wounds or natural openings, producing soft rots usually accompanied by bad smell.
- Blackfoot The agent is Erwinia carotovora and penetration is produced by the stolon, lenticels or open wounds on the surface of the tuber and from there it extends to the stem. At first, the plant presents an erect type growth, with chlorotic leaves and rolled upwards. Dark colorations appear in the vessels and inthe more advanced stages of the disease, stem rot causes the death of the plant. High humidity and temperature favor the development of the disease. The strategy to combat the disease is to maintain good drainage on the plot, harvest with the ripe potato and in dry weather, use certified seed, do not plant in wet and cold soil, do not pass in the nitrogen fertilizer, clean and disinfect the tools and farming tools, carefully handle the tubers, make wide rotations, remove waste from the previous crop and avoid transporting and storing wet tubers.
- Bacterial wilt Ralstonia solanacearum is a Gram-negative bacillus that lives in soil and has a wide range of hosts. It enters the plant through the root where it multiplies and migrates to the stem colonizing the vascular bundles which it fills with a mucilaginous substance that prevents the normal feeding of the plant; the plant withers and may die. The affected plants recover during the night due to the drop in temperatures, but during the day the disease continues its progress. If we make a transverse cut of the stem we will see how the xylem exudes some drops of grayish brown color and if we put in contact the two parts of the stem and separate them, threads of mucus appear. In the tuber appears a soft rot that if pressed exudes a few white drops. In South America the disease is known as “vaquita” precisely because of these milky exudates. The bacteria can be present in seed, soil, implements, irrigation water, nematodes, insects and animals. It is recommended to use certified seed, make rotations with other nightshades, remove weeds that serve as hosts, remove remains of the previous crop and disinfect tools and warehouses. In case of onset of the disease, the quarantine measures established by law must be applied.
- Bacterial necrosis Clavibacter michiganensis spp. sepedunicus is a short Gram positive bacillus with no mobility. Plants may be infected and not give symptoms until the end of the growing cycle; the lower leaves wither and roll upwards, subsequently appearing yellowish spots in the intererovial spaces. If we cut a stem, the vascular area has a brown color and exudes a whitish substance. In the tubers it causes a rot under the skin in the form of a ring. The pathogen survives from one cycle to another in infected tubers. The measures of struggle are the same as for bacterial wilt.
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