Solid messages from potato conference

By Andy Doyle

Good weather is likely to have been a significant factor in the relatively low turnout at last week's National Potato Conference. But there was still good solid advice on offer to the decreasing band of ever more specialised growers.

Amongst the main points was the fact that it is now essential that farm saved seed be no more than one generation from certified and that potato inspectors inspect this crop in the preceding year. On the production side any input treatments that do not directly or indirectly increase either radiation interception, radiation utilization or the efficiency of dry matter partitioning will not contribute to tuber yield. And where slug control is necessary the first of two treatments should be applied before the canopy meets across the rows.

Class X seed

The practice of saving seed by resowing tubers from the previous year's crop is traditional in potato production. However, this must now come to an end for commercial producers. From 2003 onwards a grower who plants seed potatoes other than certified seed or Class X seed will be breaking the law.

Keith Armstrong of the Department of Agriculture explained that Ireland is one of the relatively few high-grade seed producing regions in the EU and this must be protected.

Certification controls the quality of certified seed multiplication for a range of characteristics, but not all. The process involves going from initial meristem cuttings through to mini tubers and through the different field generations up to Elite seed. If this is multiplied the progeny is regarded as Class H, which is the lowest grade seed that can be sown for ware production.

The introduction of the new Class X category now prevents the progeny of Class H seed from being saved and resown on farm for ware production. Farmers now wishing to home save potatoes must sow a portion of their crop to Elite or higher grade seed and apply to the Department to have this considered as Class X seed for reproduction.

This will involve standard certified seed production checks such as eelworm in the intended field, rotation, etc. before planting. It will also involve field checks to ensure that the variety and seed being grown conforms to existing standards. And it will involve post harvest checks on both the field and the tubers. If everything is up to scratch the Department official can designate the progeny as Class X seed and this can legally be used for replanting in the following year.

Class X seed is not certified seed and so it cannot be marketed legally, it is only for home use. Keith said that it is hoped that implementation of this regulation will improve both the yield and quality of the national potato crop.

Bulking tubers

Tuber yield in potatoes is determined by the amount of radiation intercepted by the crop canopy; the efficiency with which this radiation is converted to plant dry matter and; the proportion of this dry matter which goes to form the tubers. John Burke from Teagasc, Oak Park outlined the factors, which influence these steps.

Radiation

Radiation levels vary from year to year and potatoes are not very demanding of high radiation levels. There is a clear seasonal pattern in radiation levels, according to John Burke. Radiation levels peak in June and decline thereafter. Average levels for August are similar to April but no matter how bright the days in September the radiation values rarely match those of even a dull August. So crops should be managed to target full utilization of radiation by June. Failure to do this cannot be compensated for by growing on the crop into late September/October, John said.

Leaf area governs radiation utilization and peak leaf area is influenced by variety, fertilizer application and planting date. Ground cover is normally achieved at a leaf area index (LAI) of three. When this happens a potato crop intercepts about 85% of the available radiation.

Early planting and physiologically aged seed speed up ground cover. Physiologically aging seed for 200 day-degrees at >4°C can advance emergence by about 10 days.

Nitrogen also has an impact. More nitrogen increases the amount of foliage early in the season but it is important to achieve a balance between foliage bulk and lodged foliage. Nitrogen is also important at the end of the season to prolong leaf growth.

Photosynthesis

Photosynthesis enables the conversion of water and carbon dioxide to glucose, sucrose and then starch in the presence of sunlight.

As sunlight is outside of our control and carbon dioxide is abundant in air the process is limited by water availability. When water is present in adequate amounts for uptake at the roots it is transpired up through the plant and out through the stomata on the leaf surface. So having adequate water enables the stomata to remain open and to take in enough carbon dioxide for photosynthesis.

Partitioning

During growth the dry matter produced is used to produce stems, leaves, roots and tubers. It is only the surplus that is made available for tuber production. After tuber initiation it is the strength of the competition from the canopy that governs how much is available for storage in the tubers.

Nitrogen has a big impact on canopy growth and dry matter portioning. Nitrogen increases both the amount and duration of the canopy thus adversely affecting the partitioning of dry matter to the tubers. Table 1 shows the partitioning of dry matter over time in a crop of Rooster grown at Oak Park. Canopy production is dominant in the earlier part of the season while towards the end all the dry matter is diverted to tuber production.

Variety also has a big impact on dry matter partitioning. First-early varieties like Home Guard initiate tubers when the canopy is small and divert more of the total dry matter produced to the tubers. This is critical to achieving a commercial yield from an early harvest.

In a variety like Cara tuber initiation does not commence until ground cover is achieved and full yield will only be achieved with later harvest. Rooster is intermediate in this respect.

Slugs

Andy Evans from Scottish Agricultural College outlined the current thinking on slug control in potatoes. Control means test baiting, knowing your slugs, using cultural control and then using pellets.

Firstly you have to know which slugs attack tubers. The grey field slug, which causes most damage in cereals, is only likely to attack damaged tubers. The black slug can cause severe grain hollowing in cereals but prefers to feed on the haulm of potatoes. The garden slug will attack tubers but the keeled slug is likely to cause the greatest damage. Andy said that the threshold number of slugs for damage to tubers is much lower than for cereals.

Bait

Test baiting in advance of planting is very important and this should continue all through the season. Andy said that baiting under tiles or plastic is best done using Alpen rather than using slug pellets. He said that slugs really like this food and are most likely to stay until it is all eaten. It is possible that slugs may 'go to ground' having eaten slug pellets and a false picture can emerge.

If slug damage is likely the first line of defence is to grow a resistant variety like Pentland Dell or Lady Rosetta. Varieties like Rooster, Cara and Maris Piper are regarded as having low resistance to slug damage.

Pellets

Once the crop is planted the only defence is pellet application. Andy suggested two to four applications in conjunction with continuous monitoring. It is critical that at least one pellet application be applied before the canopy closes in order to get the pellets down onto the soil. The more times pellets are applied the lower the amount of tuber damage likely and the lower the slug numbers at the end of the season.

Following trials comparing the two main actives, methiocarb and methaldehyde, he concluded that methiocarb tended to result in a more consistent reduction in slug damage. However, he also pointed out that where reduced rates are used some methaldehyde formulations have more pellets per unit weight thus resulting in more baiting points for the slugs. This can be important he said but also acknowledged that weather durability is also important for the continued effectiveness of the pellets.

Previous crop

Andy reminded those present that slug control must be a long-term issue and that monitoring should begin in the previous crop. This can enable high-risk fields to be by-passed or for control to begin much earlier.

He suggested ploughing stubble ground early and using pellets on the ploughed ground when there was little alternative food available. Intensive cultivation may also help kill some of the slugs present. Stubble cultivation in the autumn can also be very effect at reducing numbers by drying out the eggs and desiccating young slugs.

Other Points

The opening address to the conference by Minister Noel Treacy was interrupted by a Kildare IFA protest at the Government's abandonment of farmers and agriculture. A number of speakers, including Kildare county chairman, Michael Dempsey and IFA vice president Rudraigh Deacey stated that it was intolerable that meeting after meeting with Government officials offered nothing to the farming community other than taking away over €300 million of previous funding.

Bord Glas has been a significant asset to the potato and horticulture sectors since its introduction and must be allowed to maintain its independent status rather than be incorporated into the primarily food exporting Bord Bia.

Potato chairman, John Sheridan called on Minister Noel Treacy to ensure that potatoes now be included in the rules and regulations of the Groceries Act to prevent the crop being used as a marketing tool in inter-supermarket competition.