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Crop Crack12.06.10
By Wendy Fearon
After some very high temperatures and the continued dry weather during May, we are currently in a run of cooler and more showery weather, ideal for many diseases.
The recent rain has been very welcome as some crops were beginning to show signs of stress, especially on lighter soils, allowing all crops to grow very rapidly at present.
Winter barley crops range from start of flowering (GS61) to end of flowering (GS69). Rhyncho & Mildew are present but levels are low where disease control has been maintained. Ramularia will now also become a threat to the upper plant, and underlines the need to control this disease in spring barley also.
Winter wheat crops range from flag leaf sheath elongating (GS41) up to ears 50% emerged, (GS55). Disease levels vary depending on fungicide programs & timings to date, with crops ranging from very clean to those showing high levels of both Septoria & Mildew.
Later drilled spring barleys are somewhere mid stem extension (GS32-34) but be careful, those under moisture stress are actually much further on than their height suggests. The most advanced have the awns emerging (GS51-52). Westminster is still fairly clean with levels of Rhyncho low but Wagon & Quench are very much more heavily infected with a significant amount of leaf area already destroyed. Straw stiffening will already be missed in the most advanced crops and should be done now in those fields thought to be at risk of lodging later.
Spring wheats are also mid stem elongation (GS32-34), with low levels of Septoria but Mildew is present on more susceptible varieties.
Potatoes
Whilst much of the crop planted has now been sprayed, effectiveness from the residual component may turn out to be variable especially in early drilled crops.The residual partners used (AFALON/ LINUREX/SENCOREX/SHOTPUT) all depend on adequate soil-moisture to penetrate and re-distribute though the top layer of the soil to work properly. As soils have remained more or less dry for some weeks now, so their activity is likely to have been adversely affected.
Scutch, other grasses or volunteer cereals are not controlled by the above named products. Where these weeds are a problem, the graminicide FALCON can be used. This product works by contact activity only, therefore the greater the amount of leaf present at time of application, the more effective the control. Falcon can cause transient yellowing however, therefore not approved for use on certified seed. In certified seed crops the graminicide LASER (plus the wetter, FORTUNE) should be used, but is not as effective as FALCON particularly on annual meadow grass.
Changes to the NI Blight Population
Rapid changes are continuing to take place in the strains of blight fungus right across northern Europe. Results from the second year of the All-Ireland AFBI/Teagasc Blight Project show that the NI blight population has tended to mimic changes seen in the GB population in very recent years, continuing to move towards the more aggressive A2 type. From 45 sites monitored in NI last year, AFBI found that more than half the samples tested were the A2 type , and all of these were the ‘Blue-13’ strain. All A2 samples proved resistant to phenylamides i.e. metalaxyl.
This genotype is considerably more aggressive compared with those found in previous years, ie it will cause more damage faster to foliage, stems and tubers than other genotypes.
This genotype sporulates at lower temperatures than older genotypes, and its latent period of infection (the duration of time between first infection and symptoms becoming visible) is about 4½days at 13°c and 3days at 18°c, about 1½days faster than older genotypes.
The threat of blight each year is mainly determined by the prevalence of weather conditions suitable for its spread and development. The recent milder weather has triggered high risk conditions. Assuming weather conditions favour the development of blight, the presence of this more aggresive genotype has very important implications:
Outbreaks of blight are likely to occur earlier because of A2-13 activity at lower temperatures, producing larger lesions that produce more spores than most other genotypes. Hence Smith’s Periods (two consecutive days with a minimum temperature of 10°c and 11 hours of relative humidity greater than 90%) may no longer be an accurate measuring stick to predict high risk conditions. Therefore spraying should begin sooner, typically when the crop has reached rosette stage.
Because the latent period of infection is considerably shorter than conventional genotypes ie a shorter lifecycle than previously seen, so spray intervals should be kept to a maximum 7days. Any extension of interval is more likely to lead to infection than before.
A2-13 is resistant to phenylamides. Therefore independent advice (including that from SAC and BPC) is to no longer recommend phenylamide products be used to protect the crop during rapid haulm growth. The genotype is controlled by propamocarb, in CONSENTO.
There are three different ways by which the blight fungicide moves through the plant:
Contact fungicides eg. DITHANE, RANMAN, SHIRLAN, TIZCA - these products protect only the outer surface of the leaf onto which they are deposited. They do not move into the leaf and therefore do not normally protect any new growth that emerges after application.
Translaminar fungicides eg. CURZATE M WG, HARPOON, INVADER, REVUS - these products move into the leaf onto which they are sprayed and then redistribute throughout the leaf tissue as it increases in size. They become rain-fast once they are in the leaf.
Systemic fungicides eg. CONSENTO, FUBOL GOLD, INFINITO - these products move in through the leaf surface and upwards into the new growth that emerges between applications, protecting that new growth from infection also.
Products that have zoospore activity are the most effective 1st spray, applied at the rosette stage prior to rapid haulm growth. RANMAN, SHIRLAN or TIZCA will control any zoospores that may be in the soil (either from ground-keepers or infected seed) and provide good protection of the new plant.
Where crops are at the main canopy development it is essential the product being used is fully systemic to properly protect the new leaves being put on between applications. Later planted crops are higher risk as they produce more new growth between applications than earlier planted crops, at a time when the level of inoculum in the air is progressively increasing.
During the main canopy development phase the rate of new growth is extremely rapid. This places a huge uptake demand on the uptake of all nutrients, and in conditions of such rapid growth any nutrient that is limited in availability will suppress haulm growth, and as tuber initiation begins, tell the plant to form fewer tubers also. Manganese, sulphur and magnesium are three of the potentially most limiting trace elements, and timely application of these nutrients in an immediately available foliar formulation will offset this yield limiting effect.
So that the crop is not adversely affected, it is essential to apply 2-3 maintenance applications of Mn, S & Mg along with N before symptoms are seen, the first going on at the onset of tuber initiation, which normally coincides with about 30-50% ground cover.
Aphids damage potato plants directly through feeding damage and indirectly by transmission of several viral diseases, of particular importance in seed crops - aphid control is necessary on seed crops to prevent the spread of potato leaf-roll, mild and severe mosaic viruses. Aphids are the vectors for these viruses ie. it is the aphids by their feeding action which pass the virus on from one plant to the next.
A number of different species of aphids are responsible for viral spread. The very warm May has seen aphid populations increase rapidly, so seed crops should now be monitored regularly for their presence, and a control programme begun if found. When checking plants for aphids, look at both sides of leaves at all levels of the canopy, and monitor the whole crop, not just the headlands.
The crop is most vulnerable to virus transmission at the early growth stages, so it is vital to control once aphids are seen - the early sprays are much more important than later treatments in limiting virus spread.
BISCAYA controls all important aphids, including MACE resistant species, by both its contact and systemic modes of action. Central to its efficacy is its penetrant which greatly improves retention on the leaf. The oil forms a film after the water has evaporated, enhancing penetration of the active ingredient, thiacloprid, into the leaf, thereby speeding and prolonging activity. BISCAYA provides the same degree of efficacy in the mid-canopy leaves of the potato plant as on the upper leaves, still providing over 90% control 14 days after application. As a resistance management strategy its label allows only one application in ware crops and two in seed. In order to minimise the possibility of resistance developing, other aphicides with different modes of activity should be included in a control programme, an ideal partner being DOVETAIL. BISCAYA should be used first in the control programme to take out any aphids resistant to other approved aphicides.
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