The weather continues to be very unsettled. Growth has been good over the last number of weeks with most crops looking well, but spray days have been difficult to find and many crops have not been sprayed at the correct timing due to the absence of a suitable spray window. Blight is reported in many areas across the province. Rhynchosporium is present on almost all crops of spring barley. I would advise under current disease pressure to ensure spring barley crops get a robust T2 fungicide and winter wheat an earwash .SUMIALPHA should also be included at this time.
Alternating between different products with different modes of activity is an essential part of an anti-resistance strategy. As levels of blight inoculum build over the course of the season, so growers must guard against the possibility of the fungus becoming resistant to the chemistry used – overuse of any one product will increase the likelihood of this dramatically. Continue therefore to switch between products from different chemical classes that are active on zoospores, so providing the best protection against tuber blight.
Product Types and Mobility in the Plant
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, RESPLEND, 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.
Last year BASF brought to market a new active ingredient from a novel fungicide group, ametoctradin. This active has excellent activity against foliar and tuber blight and is rainfast in less than 1 hour. It moves into the leaf and is redistributed to the growing margins, so protecting the leaf as it grows in size. It also binds strongly to the waxy layer on the surface of the leaf and is redistributed over the surface onto new forming leaves. This makes the product ideal for use during the rapid canopy phase. To manage resistance, the active is co-formulated with more than a full rate of dimethomorph (relative to INVADER) and marketed as RESPLEND. The dimethomorph brings excellent curative activity to the product also and makes the product well suited right through the stable canopy phase.
Product Selection and Growth Stage
Early canopy development phase, emergence to rosette – 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. Rapid canopy development phase, end rosette to beginning tuber initiation – Because of the considerable amount of new growth being produced during this phase, it is essential the product being used is fully systemic to properly 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. Of the fully systemic fungicides, only those based on propamocarb effectively control all genotypes including the A2-13 strain. With the withdrawal some years ago of MERLIN & TATTOO, CONSENTO is the only fully systemic in the marketplace, containing propamocarb and fenamidone. The fenamidone adds a different mode of activity and builds in early tuber blight activity also.
Late canopy development, tuber initiation phase -As the crop canopy closes across the drills, the crop starts to initiate tubers. From this point on these daughter tubers are vulnerable to infection, so fungicide selection must also consider introducing tuber blight control, spread by zoospores being washed off any infected leaves or stems. At this stage INFINITO should now be introduced. Being fully systemic and translaminar it continues to protect new growth, and being curative it provides kickback should any foliar/stem blight appear. INFINITO remains one of the highest rated of the 21 products compared by the Euroblight group last year.
Canopy stable, tuber bulking phase -The need for systemic activity diminishes as the rate of haulm growth slows down during the mid season tuber bulking phase. Tuber blight activity should also become a factor as tuber development gets underway.Having tuber blight activity INFINITO is an ideal choice at this time also, but because it is limited to a maximum of 4 applications, 1 or 2 applications should be kept back to apply when severe pressure comes on, mid or late season.Other excellent alternatives at this time are INVADER & RESPLEND. The dimethomorph component of both products is curative and very persistent within the leaf and stem, so both are ideal alternatives when pressure is high. Both are translaminar, protecting the foliage as it fills out. The maximum dose for dimethomorph has been increased from 150gm/ha to 180gm/ha, both products now putting this higher rate on at each application. This has further improved the kickback and persistence of both products.
If the spray interval becomes unavoidably stretched by a day or two beyond 7 days, HARPOON should be tank-mixed at full rate along with whatever product is being used to provide the kickback effect this product gives. HARPOON is cymoxanil and is the most curative active available, meaning it will control any latent infection that may already entered the leaf as a result of the delayed application. However it breaks down within 3-4days after application, hence it must only be applied in mixture with another fungicide when curative activity needs to be maximised.
If spraying during periods of unsettled weather, rainfastness of all products will be further improved with the addition of a sticker to the spray solution. GUARD is a latex sticker and will improve the retention of the fungicide onto leaf surface, so improving both adhesion onto the leaf immediately post application, and over the following days should rainfall levels remain high.
General Comments on Good Practice
Never use any blight product at reduced rates; to do so will seriously compromise its performance, reduce its persistence, and encourage the selection of blight resistance.
As most products are either contact or translaminar only, coverage of the maximum surface area of the plant is critical. Maintain water volumes of at least 200lt/ha from the start to ensure good coverage, increasing to 300lt/ha in very leafy crops later in the season.
Make sure nozzle type used is correct to ensure sprayer pressure, droplet size and water volume applied are as per the label; these factors are as important product choice in terms of achieving good coverage of the foliage. Inspect and calibrate nozzles regularly to maintain performance. Trials have shown that fitting angled nozzles alternating to face forwards & backwards along the boom gives better coverage of the plant and significantly reduces drift.
Maintain boom height at correct height above the canopy throughout the season to ensure correct spray pattern and minimise drift; 50cm for 110° fan nozzles spaced 50cm apart.
Do not spray when leaves are wet as this will significantly increase the likelihood of runoff, so reducing the amount of active retained on or into the leaf .Make sure points and areas around line poles are fully covered. Inspect crops regularly throughout the growing season to identify any outbreak early, and determine growth stage. Use this to assist with product selection.
Trace element deficiencies
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.
The damp spring has been ideal for slugs, so there is an increased risk of slug damage this season on crops in heavier ground. The damper soil encourages slugs to come up to the soil surface, so providing the ideal opportunity for good control. The key timing for effective slug control is just before the crop canopy meets across the rows. Miss this timing and the control will be less effective regardless of how many times they are treated. Take advantage of any rainfall by applying pellets just after rain when the slugs move up onto the soil surface, Potato varieties particularly susceptible to slug damage include Maris Piper, Desiree and Kerrs Pinks