IV: Second article on physiological age.
When planting early generation seed potatoes it is important to recognize that planting design can be instrumental in reducing the spread of aphid transmitted viruses.
While planning the field lay-out for seed potato production you may want to keep virus control into consideration. The seed potato producer faces more challenges than viruses, but we'll begin with their control. In later columns we talk about fungi, bacteria, physiological age and cultural practices.
You want to take into account the insect pressure as you have noticed the last years, when you are looking for the right field. Isolation is still key, but what is isolation, how far and from where? If we want to control the spread of aphids it is paramount that we look at what aphids are doing and when. If you think that growing seed potatoes is becoming more of a challenge you may be right. The combination of pests and diseases that we are facing today creates other problems than we may have anticipated.
We have a tendency to control pests and diseases with chemicals. The cause and effect in nature are not always what we would like to see. The use of insecticides against Colorado potato beetle or leaf hoppers combined with the use of fungicides against Late Blight eliminates all predators and diseases that suppress aphids. Therefore the combination of insecticides and fungicides creates an explosion of aphids, because aphids can double their population every second or third day. Admire, applied at planting, leaves the predators in place and therefore creates a much better environment to keep potato colonizing aphids under control. On potatoes, regardless of the immediate target, never spray an insecticide that does not also kill aphids. We must continuously inspect our fields to be able to intervene with aphid suppressing chemicals such as Monitor, Provado and Fulfill. Aphids in high concentrations can suck the life out of potato plants. In high enough numbers aphids will cause black circles to appear in your field, which look from a distance like Late Blight spots. However this is not the greatest problem caused by aphids. The main reason we don't want aphids around is the fact that they are such efficient transmitters of virus.
The two most important viruses concerning us are Potato Virus Y (PVY), one of the viruses causing mosaic and Potato Leaf Roll Virus (PLRV), which can cause 'net necrosis' in Russet Burbank. Depending on the potato cultivar both kinds of viruses can cause yield reduction when occurring in large enough percentages. Seed potato producers are doing what they can to limit the percentage of virus in their crop.
The methods of control depend on the mode of transmission.
Aphids have what are described as piercing-sucking mouthparts. PVY is transmitted on the outside of the aphid's mouthparts (stylets) and has a relative short period of activity. Aphids that do not colonize potatoes can and do transmit PVY in great numbers. When aphid hosts such as small grains, sunflower and canola start to mature the aphids leave for greener "pastures" and may move to a neighboring potato crop. If any PVY-infected potatoes are in that field the visiting aphids can acquire the virus from one plant and transmit it to other healthy plants. Since these aphids do not colonize potatoes, and can't survive on potatoes, you will not find these aphids in your crop but virus transmission may have occurred. Aphid traps can be used to monitor the flight activity of such species. Spraying insecticides is ineffective, because the aphids can transmit PVY much sooner than they will be killed. When the colonizing aphids are detected you are already too late to prevent PVY transmission.
Therefore rule number one is to start with clean seed. When there is no source of virus inoculum there can be no spread. Most aphids will land on the outside rows; therefore you should plant some crop other than potatoes in the border rows. Maintain a fallow border outside the border crop, BUT NOT between the border and your seed lot. If you leave black soil around your early-generation seed that makes the outside rows of your seed potatoes into border rows (= targets for aphid landing). Also, do not leave black headlands. Plant soybeans, winter wheat, or, if available PVY-resistant potatoes around especially your high class seed. Aphids lose their ability to transmit PVY after only 1 or 2 attempts to feed. Therefore you can protect your crop against infection brought in by aphids from other fields by giving the aphid the opportunity to probe green plants before it reaches your potatoes.
In Europe the seed potato crop is covered with mineral oil, which has a cleansing effect. A virus carrying aphid will infect a plant covered with oil, but will not infect the next one. Neither will it pick up from an infected plant. Research in the US has shown that spraying the crop with mineral oil on a regular basis during the growing season can reduce the spread of PVY.
Cultivars such as Shepody and Russet Norkotah show scarcely any symptoms and do not suffer much yield reduction from PVY. However for the seed potato grower these cultivars are sources of inoculum from which the rest of the farm or neighboring farms can be infected. PVY resistance is becoming available and will be of great help to the growers.
The transmission of PLRV is very different from that of PVY. PLRV is restricted to the food transport vessels (phloem) of potato. Since an aphid typically must feed for 20-30 minutes to reach the phloem, transmission of PLRV is accomplished only by aphid species that colonize potato. Once ingested the virus moves through the insects gut and to the salivary glands. This may take 1-2 days before the aphid can transmit PLRV. But, once infected the aphid can transmit PLRV for life. Green peach aphid is by far the most important vector of PLRV. Systemic insecticides such as Admire control the spread of the disease during most of the season. with respect to within field spread, but transmission is still possible by winged aphids that arrive already capable of transmission. An aphid should die before it becomes competent vector. Canola is a host plant of the Green Peach Aphid. Extra attention is required, when the neighboring canola begins to mature. Once aphids start to colonize, typically beginning in early to mid-July, you want to make sure that any insecticides you use kill aphids. Provado and Monitor are effective aphicides. Fulfill is a promising new chemical, because it paralyses the aphid's suction pump quickly causing the aphid to stop feeding and eventually starving it death. Admire may not last the whole season, therefore inspecting the crop for aphids is a necessity. Most PLRV transmission is by colonizing aphids wandering from plant to plant. A systemic insecticide therefore helps to reduce infection.
Recap: Plant clean seed, plant away from infected lots. Rogue early. Plant early generation seed first.
Against mosaic (PVY): Keep green plants around your field to have the aphid clean itself before it enters your field. Keep the leaves covered with mineral oil.
Against Leaf Roll (PLRV): Use Admire at planting, watch later in the season for colonizing aphids and use only insecticides that also kill aphids.
Plant early, kill early. Don't go for the biggest yield, if you want clean seed.
At the Potato Expo Conference in February, 1999 in Fargo, ND, potato growers were given conflicting recommendations such as when to top kill. One speaker suggested to kill the crop late in the season as short time as possible before harvest to reduce silver scurf and Rhizoctonia, and the next speaker recommended to top kill early to reduce virus infection. It is not surprising to hear growers ask what they are supposed to do. At the time of planting the same opposing recommendations are given concerning planting: To obtain optimal stand do not plant too early, to avoid virus infection plant as early as possible.
Obviously growing a good seed lot is a compromise between the different priorities.
In this column I would like to look at the management practices that used for disease control in seed potato production. Growing a good seedlot starts with the choice of land, quality seed and full season management. Grading seed potatoes for fungal and bacterial diseases does not guarantee freedom of those diseases. A good seed lot is produced by hard work and will always be easily recognizable.
Cutting seed gives ample opportunity for disease organisms to enter tissue, which is why we want seed to suberize as soon as possible. Planting in cold soils delays suberization and reduces sprout growth. Therefore disease organisms have more opportunity to infect, before the sprouts have grown out into plants strong enough to survive on their own roots. Planting freshly cut seed in a moist warm (not hot) soil generally does surprisingly well, because that is an environment ideal for suberization. However many people would not push their luck this way. They will try to protect their seed with seed treatment. Most seed treatments have worked well, but, when late blight occurred in areas where it had not been a problem before, the situation became confusing. It is only logical that growers want to plant seed that is free from late blight in order to avoid seed born late blight infection during the growing season. However, almost all seed tubers visibly infected with late blight will usually rot in the ground, or their sprouts will not emerge. Therefore such tubers will not produce a plant. Nonetheless when seed is precut or treated with a fungicide without a specific anti-late blight component some seed pieces may produce sprouts that emerge and will become a source of infection.
Thanks to the work of Lambert in Maine and others, we now know that late blight spores can spread and infect tubers at the cut surface. This seed infected with spores during cutting may be healthy enough to produce a plant that will give spores. Thus establishing a source of inoculum. Therefore the new seed treatments that will control Late blight such as MZ-Curzate and Topsin-MZ-Curzate are so welcome. The fungicides with the Curzate component has given close to 100% control in trials of Secor and Gudmestad from North Dakota. The other seed treatment products in the same trials such as Maxim, Tops and Mancozeb gave insufficient control. Maxim in the order of 20 - 30%, Tops around 50% control and Mancozeb somewhere close to 80%. Adding Mancozeb improved control by Tops and Maxim. The addition of Curzate gave the best possible results.
When late blight became a problem in new areas, due to the occurrence of more severe strains, growers wanted to delay the onset of the disease by restricting the movement of seed. Unfortunately even though the cleanest possible seed may be perceived as a safeguard, only spraying preventatively against Late blight will keep the crop healthy and even then Late blight can occur in the storage. Dry weather is the best help.
Anybody, for whom late blight is a new disease, contaminated seed tubers are blamed as the first source of inoculum. Unfortunately it does not matter what or where the source of inoculum is, a strict preventative spray schedule has to be adhered too. Crops which are not sufficiently sprayed, cull piles and volunteers become the source of inoculum. There are indications that late blight spores can move for miles, but most of the time an infection source is close. Some states have adopted regulations to have cull piles removed while other states have plant disease control regulations in place that can be used to force farmers to cover or bury their cull piles.
Late blight is on the mind of most growers who have witnessed its devastating effects, but, dry rot, Rhizoctonia and Silver scurf are all diseases we want to protect against as much as possible right at planting. Seed treatments such as Maxim and Topsin control seed borne diseases, but not inoculum already in the soil. External circumstances such as temperature and moisture are also important.
The most important remedy against any of the rot causing organisms is bruise prevention. To avoid bruising the tuber temperature during harvest should not be below 45F. However to keep the occurrence of Leak and Pink Rot to minimum the tuber temperature during and after harvest should not be above 65F. Here we are facing the inconsistency again. Kill early to reduce virus infection, but do not harvest too early in the season when the tuber temperature is above 65F. Under those circumstances, harvest early in the day. Anyone harvesting with a tuber temperature above 65F has a good chance to see his crop break down in storage. To reduce infection with Silver scurf and Rhizoctonia keep the period between topkill and harvest as short as possible. Storing seed as cool as possible is the standard technique to reduce the spread of diseases, but warming up the seed before handling and cutting will improve the stand.
During and after harvest spores of diseases such as Silver scurf and Late blight can spread to other tubers. The use of Chlorine Dioxide in and out of storage is recommended to kill as many spores as possible and reduce their diffusion. Although one must never forget that the most ideal conditions for disease development are free water. Getting rid of free water is the most important first step in storage management.
Somewhere a compromise has to be found in when to plant and when to harvest. In reality most seed potato producers find that compromise every year, because the weather often dictates when what may happen.
Physiological age of seed potatoes is one of the topics, which is often discussed with potato growers. Because the growing season can influence how the seed will perform, this article about physiological age seemed timely.
The history of the seed potato you produce starts very early in the growing season. From tuber initiation the tuber can be influenced, physiologically, by outside conditions. All of this may have an effect on the performance of that tuber as a seed tuber the next season.
Tubers age from the moment they start to grow. One of the most important factors influencing physiological age is temperature, but it is not the only one. Other influences are variety, moisture, stress, bruising, cutting etc. Any stress advances the physiological age.
The different stages of physiological age are dormancy, rest, apical dominance sprouting, multiple sprouting, weakened sprouts, little tuber phenomenon or tuberization.
Dormancy: During the growing season and just after harvest, the tuber does not sprout under moderate circumstances. This period is called dormancy. When the exception occurs, i.e. sprouting during the growing season under temperature and moisture stress, we recognize it as heat sprouts. In other words in such a case the dormancy is broken.
During storage, the tubers are stored at temperatures around 36-38F, to extend the dormancy period. At the beginning of the storage period the tuber does not sprout regardless of the temperature, but at a certain moment the tuber would sprout if the temperature were at e.g. 65F. When low temperatures keep the tuber from sprouting the period of rest starts. This is called by the French the period of latent sprouting.
Apical Dominance: If the tuber is held at moderate temperature levels the first sprout will be an apical sprout which signals the period of apical dominance.
Multiple Sprouting: The tuber may skip apical dominance if it is held at low enough temperatures and is warmed up with a sudden heat shock during the period of latent sprouting. Then the period of multiple sprouting has started. There are different ways to achieve multiple sprouting; cutting the seed or warming the seed suddenly during the period of latent sprouting. Another method is removing the apical sprout.
Senility: When seed becomes older the sprouts will become weaker and the tuber enters the period of senility.
Little Tuber Syndrome: After the period of senility, the tuber will form small tubers directly on the sprouts and the seed tuber is lost.
From the period of harvest we are better able to manipulate the temperature, and therefore the physiological age of the seed potato. To keep seed from aging we want to cool the storage as quickly as possible, following the initial wound healing period. When the harvest is early and the fall is warm, aging of the seed will have occurred in the field and will be expressed in the spring by the fact that the tuber has a tendency to sprout early.
The physiological age of the seed potato determines its yielding ability. In short: a seed tuber that is still dormant in cool soils will produce few sprouts often only an apical dominant sprout. It will produce a one-stem plant which will have a small number of large size tubers. Emergence of the plant is late compared to physiologically aged seed tubers.
Cut seed and older whole seed will produce multiple stems and a larger tuber set. The time of emergence will depend on whether the sprouts are broken off or not and on the soil temperature, because both low and high soil temperature age seed.. This stage in the physiological age is optimal for the yielding capacity of the seed, if the growing season is long enough.
Between the beginning stage of multiple sprouting and senility is a stage, which is reached by e.g. chitting, or presprouting. Exposing seed tubers to indirect light will harden the sprouts and advance the physiological stage. Planting such seed will give an earlier emergence, and earlier maturity. This is especially welcome, when the growing season is short, such as early table and for seed production, where a larger number of smaller tubers is desirable. Maturity reduces the susceptibility to virus and is therefore something a seed potato grower tries to enhance in his crop. Earlier emergence will enhance the yielding capacity of the seed, when early frost or early topkill is common. A less effective method, but much more practical is warming the tubers until one sees the white sprouts pipping.
Seed potatoes in the stage of senility should not be planted. There is a good likelihood that the seed will not emerge from cold soil, because the stress of the cooler temperature will enhance the physiological age to the stage of the small tuber syndrome, where small tubers are formed without the development of a plant. If tubers in the stage of senility are planted in warm soils the emergence will be quick; the plants will remain small. The yielding capacity of such seed is limited.
The right physiological age is important for the yielding capability of the seed. Besides the variety, temperature and stress during the growing and storage season are factors influencing the yielding capability of the seed, because they influence the physiological age.
The most optimal physiological age of the seed depends on your growing season and the destination of your crop. In the next issue it is my intention to go into more detail on how to use physiological age to your advantage.
East Grand Forks, MN
The basis of the physiological age of the seed you produce for next year is determined during this growing season. Last month, we discussed the stages of the physiological age of the seed tuber. This time, we look at how physiological age can be used to optimize the expected performance of your seed crop.
As we discussed last month, physiological age of seed potatoes has an influence on the stem numbers and therefore on the number of tubers. What is also important is its effect on the timing of maturity. The length of the growing season and the final use of the crop, determines which physiological age is most advantageous to the grower.
The factors that can influence physiological age are: variety, temperature during growing season and storage, stress, bruising, de-sprouting and humidity. The cumulative temperature during the growing season is something that cannot be manipulated. However it is good to know the temperatures during the growing season compared to earlier years. This is a reason why it may be advantageous to buy seed potatoes from the same seed potato producer over the years. A seed potato producer can keep records on weather conditions and temperatures and how they compare with earlier years. This is how one can develop an expectation on how advanced the physiological age is of the seed will be. If the storage climate has been the same, a grower may be able to predict (or interpret) the tendency of these seed potatoes to sprout. We may get to a point where we can adjust the storage climate according to the situation of the preceding growing season. However we are not that far along yet. In any case a seed potato buyer may use this information to determine whether to keep the seed cool as long as possible or warm it up early. Early warming up may result in a need to de-sprout. Not all varieties withstand de-sprouting equally well, besides the increased opportunity for disease infection, but in general de-sprouting will result in more stems and smaller tubers.
A way to determine the physiological age of the seed, other than by its history is to take a few tubers and keep them at room temperature. The speed of sprouting and the number and shape of the sprouts gives a good indication of the stage of the seed potatoes.
Processing stock:
Physiologically young seed, just after apical dominance, produces few but strong stems, creating a plant that grows for a long time before reaching maturity. For processing stock, where large size tubers are required, young seed in the early stage of multiple sprouts is ideal, because it allows tubers to develop during a long growing season into large tubers. The expression "Northern Vigor" which has been used by the northern states for years and others too, basically means physiologically young seed, producing a crop of which the maturity is as late as possible under the circumstances.
A variety such as Russet Burbank has a tendency to produce a large number of stems. Therefore a processing grower would like to keep the number of stems low. Young seed, not de-sprouted is expected to produce the best results. A variety such as Shepody, where the small number of tubers often is a problem, the grower may look for warming up the seed earlier in order to have seed potatoes of which the physiological age is a bit more advanced.
Brief: For processing purposes the optimal yield of a variety that has a tendency to produce a large number of tubers, may be obtained from physiologically young seed. In a variety with a tendency to produce few stems the physiological age may be more advanced. The determination of whether the seed has the optimal physiological age can be best made by comparing the history of the crop in comparison with earlier years. A long term relation ship with your seed potato producer may be in your best interest. As said before, a sample of tubers held at room temperature will also give an indication of the stage of the seed potatoes.
Fresh Market:
Potatoes for the fresh market do not have the same size requirements as processing stock. This means that a slightly advanced physiological age may not have any negative effect on the marketable yield of the crop. For example, a crop that is to produce red B's obviously has a different requirement. Again it is good to know the history of the crop, the stresses and the temperatures during growing and storing to be able to treat the seed optimally. Seed potatoes for the early fresh market often should have a physiological age similar to seed potatoes, where one wants an early emergence and an early crop.
Seed Potatoes:
Seed potatoes need a good set, an early emergence and an early maturity. Two major components of physiological age, determining the quality potential of the seed are the number of stems which is related to the number of tubers and the timing of maturity. Since topkill is early, the seed should be physiologically advanced. Smaller tubers are for seed. Therefore, increasing the set, will result in a better yield of seed size tubers. Physiologically advanced seed produces a crop with an earlier maturity, besides a larger number of stems. This is important, because for most varieties it makes the plant less susceptible to viruses, later in the season. We do not like seed in the earliest stage of multiple sprouting. Again it is good to know the history of the seed, to know whether the seed needs to be warmed up early or whether that should be done as late as possible. De-sprouting physiologically young seed generally will produce more sprouts, more tubers, but smaller size. On smaller farms green sprouting of seed potatoes has been used, but that does not seem to be feasible on the large farms in North America. Besides, the growing season in North America is generally too long to create sufficient advantages for green sprouting. However for precious green house tubers it may be something to consider.
Brief: For seed potato growers it is important to know how last years temperature averages compare to this year. This will determine whether he needs to keep his seed cool as long as possible or warm it up early.
Keeping notes
Because the factors influencing physiological age are still so unquantifyable, the only way to predict anything is by comparing to other years. Temperature, drought stress and maturity, all influence the physiological age of the crop going into storage. Ventilation and refrigeration can manipulate the physiological age in storage. However the most optimal temperature regime for Russet Burbank is not necessarily the best one for other varieties such as Shepody. The only way, up until now, is to compare with other years. A long term deal with seed potato growers, where these comparisons can be made may well result in obtaining seed with the most optimal physiological age for the commercial growers. It is not an exact science yet, but we can improve over what is being done at this date.
Soil temperature at planting:
Physiological age also is important in relation to the soil temperature. When the soil temperature is low, sprouting is delayed especially with very young seed and the result may be a late emergence. That's when Rhizoctonia and Fusarium have ample opportunity to attack the sprouts, resulting in missing plants and decreased yield. Therefore one may want to be patient when planting physiologically very young seed and wait for the soil to warm up to the pulp temperature of the seed. The soil temperature should be the same as the temperature of the seed, because warm sandy soils under irrigation create favorable conditions for seed piece decay. On the other hand when seed is very advanced in physiological age, cold soils may result in the little tuber syndrome, where no yield can be expected at all from such seed.
Recap:
Now is the time of the year for seed potato producers to pay attention and keep notes on what happens to the crop, especially temperature, drought stress etc. Comparisons with earlier years will assist in predicting what the physiological age of the seed potatoes may be from time of harvest till planting. It will help interpret what is the age and stage of the seed.
East Grand Forks, MN