From the magazine: No. 1 2016
Category: Specialist consultations
B.V. Anisimov, S.N. Zebrin, V.N. Zeyruk,
All-Russian Research Institute of Potato Farming named after. A.G. Lorja
In the current practice of quality control and certification of seed potatoes, tuberous rots are usually divided into two main types - dry and wet.
Of the dry rots, the most common are Fusarium dry rot and Phoma rot. Often, superficial dry rot can also develop on tubers when they are affected by Alternaria.
The development of wet tuber rot most often occurs due to the transfer of infection from plants infected with late blight or blackleg to tubers of a new crop. When growing potatoes in heavily moist soils, rubber rot may develop on the tubers during or immediately after harvesting. High soil moisture during the growing season also creates favorable conditions for pink rot of tubers, and hot weather during the tuberization period can contribute to the development of watery wound rot of tubers soon after harvest.
In some cases, “mixed rots” can be very harmful: late blight-bacterial, fusarium-bacterial, fomosa-bacterial. The penetration of fungal and bacterial infections into tubers and the development of rot are facilitated by damage from nematodes, wireworms and insect pest larvae. In unfavorable conditions for harvesting and storing potatoes, the causes of the development of tuber rot can be hypothermia and freezing of tubers.
Dry rot of tubers caused by fungal phytopathogens
Fuzarioz (Fusarium spp.)
Infection can occur through seed material and soil. Damage promotes the development of rot, especially when sorting at elevated temperatures.
Fomoz (Phoma spp.)
The source of infection is mainly contaminated seed material; the infection can spread with rain. Tubers are often infected during harvest, but Phoma rot usually develops after harvesting and post-harvest sorting and/or at low storage temperatures.
Alternaria (Alternaria spp.)
Alternaria spores survive on potatoes or other organic matter in the field or directly in the soil.
Wet rot caused by pathogenic fungi and bacteria
Fitoftoroz (Phitofhthara infestans)
Spores from the tops infect tubers in the soil. Tuberous late blight can be observed during harvesting and continue to develop during storage. Damage to tubers during post-harvest processing often contributes to this.
Pink rot (Phytophora erythroseptica)
Infection occurs through the soil. The development of infection is promoted by high soil moisture and temperature. Rot develops during or shortly after harvesting.
Rubber rot (Geotrichum candidum)
The source of infection is the soil. The development of rot is promoted by strong soil moisture and warm conditions during the pre-harvest period. Proper soil drainage and storing tubers from flooded areas of the field separately from the rest of the crop can reduce the spread of rot.
Watery wound rot (Pythium spp.)
Source of infection: soil. Infection of tubers occurs through wounds. Rot quickly spreads on freshly dug tubers whose skin has not yet hardened. Warm weather during harvesting promotes the development of rot.
Black leg (Dickeya/Pectobacterium spp.)
The source of infection is mainly infected seed tubers, but in the field the infection can be transmitted from infected plants to healthy water droplets containing bacteria (raindrops/aerosols) as well as insects. Contact infection can occur from contaminated equipment or containers. Infection by these pathogens and the development of the disease are favored by moist growing conditions, but are more favorable for Pectobacterium are cool and wet conditions, and for Dickey – warm and humid.
Ring rot (Clavibacter michiganensis ssp. sepedonicus)
The source of infection is contaminated seed material. Tubers of some varieties may be asymptomatically infected. Bacteria are also spread by contaminated equipment, especially cutting equipment. In most countries, it is considered a quarantine disease; in the event of an outbreak, contaminated material is removed and disposed of from circulation.
Rot from hypothermia and freezing of tubers
Causes: low temperature (below 1 °C) before harvesting or storage. Tuber damage can also be caused by a rapid change in temperature (not necessarily below freezing). It is necessary to harvest before frost and avoid excessive cooling in storage.
Rot from damage to tubers by nematodes, wireworms and insect larvae
Potato stem nematode – Ditylenchosis (Ditylenchus destructor)
Nematodes are mainly transmitted along with infected seed tubers. It is necessary to use healthy certified seed material and exclude fields where outbreaks of the disease have previously been noted. Nematodes are difficult to get rid of because they live on so many plants. Reducing their number can be facilitated by the use of grains in crop rotation in combination with effective weed control.
wireworm (Agriotes/Tandonia/Arion spp.)
The larvae eat small superficial or deeper passages in the tuber. The passages are always narrow (unlike the damage that slugs cause), but can be highly branched. Damage caused by wireworms makes it possible for other pathogens to penetrate the tuber, which can cause various types of rot.
Along with wireworms, tuber rotting of the dry or wet rot type (depending on storage conditions) is often caused by damage from beetles, cutworms, slugs, and potato moths.
Khrushchi (larvae) eat away cavities in tubers. Unlike cutworms, they do not leave peel residue around the edges of the cavities.
Scoops (caterpillars) gnaw cavities of various sizes in tubers. Along their edges there are remnants of the peel in the form of a fringe.
Slugs
They eat away cavities of different sizes in the tuber pulp, which can facilitate the penetration of phytopathogens into the tuber, causing various types of rot.
Potato moth
It bores narrow (2-4 mm) passages under the skin or inside the tuber. A characteristic sign of moth damage is the presence of excrement on the surface and in the passages inside the tubers.
The intensity of development of tuber rot is largely determined by the degree of spread of diseases during the growing season and harvesting of potatoes. Therefore, it is important to monitor sources of infection through field surveys during the growing season and the integrated use of special preventive and protective measures during soil cultivation, preparation of seed material for planting, plant care and harvesting.
Of the preventive and protective techniques, the most effective are: cultivating potatoes in crop rotation using previous crops that clear the soil of pathogens; rational use of organic and mineral fertilizers, microelements and calcareous materials that increase the resistance of plants and tubers to diseases; using only healthy tubers for seed purposes, heating seed potatoes and then discarding infected material; disinfection of seed tubers before planting; implementation of all plant care and weed control techniques that contribute to the production of healthy, well-developed plants that are able to take full advantage of the natural resistance response to harmful microorganisms.
As a preventive measure on seed plantings, it is especially important to remove diseased plants - sources of infection - by carrying out thorough phyto-cleansing. Symptoms of diseases on plants appear at different times, so the greatest effect is usually achieved by cleaning three times.
The first cleaning is carried out shortly after the emergence of full shoots, when the plants reach a height of 15-20 cm. At this time, it is especially necessary to remove bushes affected by blackleg. The sooner diseased plants are removed from plantings, the fewer sources of possible spread of infection remain in the field.
The second cleaning is carried out during flowering. During this period, varietal impurities are usually removed, as well as stunted plants affected by bacterial and viral diseases. Usually, after the second cleaning, field testing is carried out and the compliance of the plantings with the regulatory requirements of the standard established for various categories and classes of seed potatoes is determined.
The third cleaning is carried out before pre-harvest removal of tops. During this period, remaining impurities are removed, as well as plants showing signs of bacterial (ring rot) and viral diseases.
Cleaning should be carried out by well-instructed workers in the presence of an experienced specialist who has practical skills in recognizing the symptoms of diseases and potato varietal impurities. In this case, usually two people walk along the furrow and carefully examine the plants in two rows to the right and left of the furrow along which the passage is made. Detected diseased plants or varietal impurities are dug up with a shovel along with tubers, including mother tubers, and removed from the field. It is not recommended to pull out plants, as this may leave the mother tubers in the ground, germinate again in the same year and again produce diseased plants. The tops and tubers removed during cleaning must be completely destroyed.
If there is a threat of the development of late blight and alternaria to a moderate or severe degree, a complex of chemical and biological preparations is used to spray plants during the growing season. These techniques make it possible in the future to significantly minimize losses from rot during potato storage.
An important technique that prevents infection of tubers during harvest and reduces the risk of developing tuber rot is pre-harvest removal of tops. It is carried out on seed plantings 14 days before harvesting, and on commercial plantings at least 7 days before harvesting. When the tops are removed immediately before harvesting, the tuber peel does not have time to strengthen and is severely injured by harvesting machines, which can cause massive infection of potatoes with dry and wet rot. So, for example, if the degree of development of late blight on plants has reached 50% and the weight of the crop is no longer increasing, the tops should be immediately destroyed so that the tubers do not become infected in the soil. But even in this case, it is necessary to maintain an interval between the destruction of the tops and harvesting.
The tops can be destroyed by mechanical mowing with the obligatory removal of plant matter from the field, since the affected tops are a serious source of pathogens of late blight and tuber bacteriosis before and during harvesting. It is recommended to use chemical desiccation on seed plots. For this purpose, potatoes are sprayed with Reglon Super (2,0 l/ha). The consumption rate of the working fluid must be at least 300 l/ha.
During the period of harvesting, transporting potatoes and storing them, it is recommended to systematically disinfect containers, vehicles, sortings, etc. with a 2-3% solution of copper sulfate. All potato remains after sorting and sorting are disposed of, and the equipment is disinfected with a 5% solution of copper sulfate.
In the fight against rot, all methods that prevent mechanical injuries to tubers during harvesting, sorting, transportation, and storing potatoes are effective. To do this, it is necessary to correctly adjust harvesters, potato diggers, sorters, and carefully handle tubers, preventing them from falling from a great height. The permissible height of tubers falling onto a metal surface (springy thin-sheet) is 50-80 cm, a solid wooden one - 25-50, a wooden lattice surface - 15-25, a rubberized one - 50-75, soil - 200, on potatoes - 100-125 cm.
Post-harvest control and techniques for reducing losses from rot during potato storage
No later than a month before storing potatoes, the storage facilities are cleaned of soil and old tubers, disinfected with lime with the addition of 2-3% copper sulfate, then the storage walls, ceiling, bin walls and panels are whitened with lime. Fumigation with the preparation whist is also used (bulk checkers 150-200g/1000 m3 room for potatoes).
During post-harvest control, tuber tests are carried out to identify diseases that have appeared on tubers.
The procedure for sampling for tuber analysis and regulatory requirements for the quality of seed material and commercial (food) potatoes are determined by the standards: GOST R 53136-2008 “Seed potatoes. Technical specifications"; GOST R 55329-2012 “Seed potatoes. Acceptance and methods of analysis" and GOST R 51808-2001 "Fresh food potatoes, prepared and supplied."
Tables 1 and 2 present the regulatory tolerances of tuber rot standards for batches of seed potatoes entering trade in the EU countries, Canada, the Russian Federation and the Republic of Belarus.
Most countries producing and exporting seed potatoes in their national standards usually introduce more stringent tolerances in comparison with the regulatory requirements of the UNECE international standard, especially with regard to wet rots caused by pathogenic fungi and bacteria [6] (Table 1).
Table 1. Regulatory tolerances of tuber rot standards for different classes/generations of seed potatoes marketed in EU countries
Countries | Tolerances for seed potato classes, % | ||||
S | SE | Mon 1-3 | A-1 2 | B | |
EC1 | 0,5 | 1 | 1 | ||
United Nations | 0,2 | 1 | 1 | 1 | 1 |
Germany | 0,5 | 0,5 | 0,5 | 0,5 | 0,5 |
Holland2 | 1-4 tubers per 50 kg | ||||
Finland | 0,5 | 0,5 | 0,5 | 1 | 1 |
France | 0,1 | 0,2 | 0,2 | 0,2 | 0,2 |
Belgium | 0,5 | 0,5 | 0,5 | 0,5 | 0,5 |
Denmark | 0,1 | 0,1 | 0,1 | 0,1 | 0,1 |
Bulgaria | 0,5 | 0,5 | 0,5 | 1 | 1 |
Чешская республика3 | 1,0 (0,25) | 1,0 (0,25) | 1,0 (0,25) | 1,0 (0,25) | 1,0 (0,25) |
Canada4 | 1,0 (0,1/0,5) | 1,0 (0,1/0,5) | 1,0 (0,1/0,5) | 1,0 (0,1/0,5) | 1,0 (0,1/0,5) |
1 – According to EU Directives 2002/56 and 93/17;
2 – For wet rot, 1 tuber per 250 kg is allowed;
3 – The indicator for wet rot is indicated in parentheses;
4 – Shipping/destination wet rot tolerance is given in parentheses.
The regulatory requirements of the current national standards in Russia and the Republic of Belarus for the OS category are quite comparable with the international standards of the UNECE standard. At the same time, the tolerances for batches of seed potatoes of the EC and RS categories significantly exceed the norms of the UNECE standard for dry and wet rot, which leads to a decrease in the quality and competitiveness of elite and reproductive seeds of domestic production. Currently, as part of the development of a new draft Interstate Standard of the EAEU member countries for interstate supplies of seed potatoes, it is envisaged to introduce more stringent tolerances for dry and wet rot, which will be quite comparable with the standards of international analogues (Table 2).
Table 2. Regulatory tolerances of standards for tuber rot for various categories of seed potatoes entering trade in the Russian Federation and the Republic of Belarus.
Standards | Class/Generation Norms* | ||
OS | ES | RS 1-2 | |
GOST R-2008 | 0,5 (0) | 2 (1) | 2 (1) |
GOST of the Republic of Belarus | 0,5 (0) | 2 (1) | 3 (1) |
Interstate standard (draft) | 0,5 (0) | 1 (1) | 1 (1) |
* OS – category of original seeds; ES – elite seeds; RS – reproductive seeds. The indicator for wet rot is indicated in parentheses.
In accordance with GOST R 51808-2001, for all classes of fresh food potatoes, prepared and supplied, the presence of tubers affected by wet, dry, ring, button rot and late blight, as well as frostbitten and with signs of “suffocation” is not allowed. Before conducting tuber analysis of selected samples in order to activate phytopathogenic fungi, bacteria and stem nematodes in the tubers, it is recommended to keep the tubers at a temperature of 10-20оC within 20 days.
First, the sample is weighed, then free soil and other impurities are separated. The amount of impurities is determined by weight as a percentage of the total weight of tubers of a given sample. After removing impurities, each tuber is washed in water and inspected. Non-standard and defective ones are identified and grouped by type of damage (diseases, pests, mechanical). The number of diseased tubers is expressed as a percentage of the total number in the sample. Based on the analysis data, batches of seed potatoes are assigned to the corresponding categories of seed material, and batches of ware potatoes are assigned to the corresponding classes of early or late potatoes (Extra, first or second classes).
To determine diseases and defects inside (black leg, ring rot, late blight, phomosis, darkening of the pulp, glandular spot, hollowness, ditylenchosis), 100 tubers per sample are cut in the longitudinal direction. If diseases or defects are found, the remaining tubers of the sample are also cut.
If there are several diseases on one tuber, one of the most harmful ones is taken into account in the following order: ring rot, black leg, late blight, fomoz, dry rot, ditelenhoz, suffocation, frostbite, common scab, rhizoctonia, powdery and silver scab, mechanical damage.
Tubers affected to any degree by late blight, dry rot, wet rot, black leg, ring rot, phomosis, and stem nematodes are considered sick. Based on the results of the tuber analysis, a tuber analysis report is drawn up, which indicates the number and percentage of diseased tubers.
In order to reduce losses from rot, seed potatoes from fields where late blight, phomosis, bacterial diseases have developed strongly, and tubers have received mechanical damage, during storage and in the initial period of storage, disinfection should be carried out against this infection and the causative agent of fusarium dry rot (when harvesting with potato combines this intake is required) using the drugs Maxim (0,2 l/t) or Fitosporin (1 kg/t).
Disinfection of potato tubers is carried out in the fall using aerosol generators of various types, which are mounted on conveyor loaders or sorting points. Working fluid consumption is 3-5 l/t. With this water consumption, there is no need to dry the potatoes additionally. Fumigation with the preparation whist is also used (bulk checkers 5-10 g/t)
The preparations are most effective if they are used no later than 3 days after harvesting potatoes, or better yet, immediately after harvesting when storing using direct-flow technology. When using them, you must follow safety rules when working with pesticides.
In the first 20-25 days of temporary or permanent storage (treatment period), the temperature should be maintained at 15-18оC and relative humidity 90-95%. This contributes to faster healing of injuries on tubers. The height of the tuber mound depends on the type of storage and whether it is equipped with active ventilation and climate control systems.
After completion of the treatment period, the temperature in the potato mass is gradually reduced, but not more than by 0,5-1оC per day for a period of 26 to 30 days, and is maintained during the main storage period within 2-5оC, varying somewhat depending on the biological characteristics of the varieties.
Optimal storage conditions are ensured through ventilation, cooling with outside air or a mixture of it with storage air. In all cases, the supplied air must have a positive temperature. In spring, the optimal mode is maintained by ventilation at night and in the morning for a longer time than in winter.
The temperature of the air or air mixture supplied to the potato embankment must be positive, but lower than the temperature in the potato mass by 2-5оC. The storage temperature in the storage facility should be equal to or higher than the temperature in the potato mound, but not more than 1оC.
Maintaining temperature and humidity storage conditions is achieved by ventilating the potato mound 2-3 times a week for 30 minutes.
Practice has shown that the recommended storage regime for potatoes can significantly slow down the development of tuber rot and significantly reduce storage losses.
Picking potatoes in winter is undesirable, as it can contribute to the overinfection of tubers with dry rot and, consequently, increasing the severity of the disease. Tubers affected by dry rot must be collected and removed from the top layer of the embankment. Detected pockets of wet rot must also be carefully removed along with the adjacent layer of healthy tubers.
Potatoes are sorted completely if more than 10% of the tubers are affected by fungal and bacterial diseases.