You are here

Share page with AddThis

Alternaria and Aphids - Potato Agronomy Update

Product Update
09.06.2017
Alternaria infection spreading on the leaf
Alternaria programmes should commence before disease symptoms, such as these, break out on the leaf - be aware of high risk situations with susceptible varieties or crops under stress

With AMISTAR now having a foliar label approval for Alternaria control, it is worth considering where this fits into the programme best, writes Syngenta Potato Technical Manager, Douglas Dyas. 

Whilst growers are familiar with using AMISTAR at planting, for improved skin finish and marketable yield, the product’s label now states that it can be used for moderate control of Alternaria solani. A 0.5 litre per hectare dose can be applied up to a maximum of three times, with each application providing 125 gm of azoxystrobin. This compares favourably, in terms of active applied, to the EAMU for OLYMPUS, which also contains chlorothalonil.

Of the two main Alternaria species  - A. alternata and A. solani - we know that strobilurins, such as azoxystrobin in Amistar, are more effective at controlling A. solani. The other key active to tackle Alternaria, Difenoconazole, in AMPHORE PLUS, is more effective at inhibiting mycelial growth and controlling both main species of Alternaria, but particularly A. alternata.

Douglas Dyas

In 2015 and 2016 the results from infected leaf samples sent to NIAB showed that A. solani was not first seen until around mid-July – consistently a couple of weeks after A. alternata. AMPHORE PLUS therefore fits well into the start of an Alternaria control programme, when initial infection is more likely to be by A. alternata, with AMISTAR following in the programme to bolster the later infecting A. solani.

Whilst Syngenta are working with the industry to help develop a prediction model to improve the accuracy of spray timings for Alternaria control, the usual commencement is around the last week of June/first week of July. It is vitally important to commence the programme in a preventative manner, so if at all possible a grower/advisor should not need to ‘react’ to seeing Alternaria symptoms. Risk factors, such as known susceptible varieties, stress from drought, nutrient deficiencies etc, should also be taken into consideration beforehand. Infection can quickly spread to complete early crop senescence - with resulting yield loss and effects on tuber quality (below).

Alternaria leaf die back

AMPHORE PLUS, applied at a rate of 0.6 l/ha, provides a robust 150 gm of difenoconazole, which is 25 gm per hectare more than competitor solo products. AMPHORE PLUS has the added benefit of providing a full 150 gm of mandipropamid when applied at 0.6 l/ha, so in the earlier part of the season you are in no way compromising late blight prevention whilst also focusing on keeping Alternaria out. 

Trials have shown consecutive applications of Amphore Plus in ‘a block’ to be the most effective control strategy. Foliar applications of AMISTAR should be applied ‘later’ in the season compared to AMPHORE PLUS.

If for example a grower has - taking into account CAA guidelines - applied the maximum of 3 x AMPHORE PLUS applications commencing on the 1st July at 7 day intervals, this would take them to the 22nd July. By this point there is a greater likelihood that A. solani is creeping into crops, so they could then justifiably use AMISTAR immediately after (in mixture with appropriate late blight material).

The maximum number of 3 x applications would take them to the 12th August. If the variety was due to be lifted late  - October onwards - or it was particularly susceptible to Alternaria (e.g. Markies, Arsenal etc) then the gap between the AMPHORE PLUS applications ending and AMISTAR applications commencing could be extended.

AMISTAR has an excellent compatibility profile both physically and biologically and so can be mixed with an appropriate partner for added late blight protection.

Pest Pressure in your crop
With potato crops slowly but surely progressing, all crops will need constant monitoring and insecticide applications when thresholds are hit. Thresholds for aphids are ‘if detected on seed crops and ware crops located near seed crops’. For ware crops isolated from seed crops, aphid control is only necessary when reaching five aphids per true leaf.

Most aphids’ peak flights in the summer will be the end of June/early July, then a second peak later in September. With the first peak occurring with younger plants, which are generally more susceptible to non-persistent viruses and persistent viruses having more time to rob yield, this will be the critical flight to control.

Myzus persicae on potato leaf

Key aphid vectors to look out for are Peach-Potato Aphid (see image above and below), Grain Aphid, Mottled Arum Aphid and Willow-Carrot Aphid. Also the Pea Aphid has become more of an issue, due to it in recent years being upgraded in its PVA transmission to 0.7. 

Myzus persicae colony

Viral symptoms for Potato leaf roll virus (PLRV) is as the name suggests leaf rolling, but it can also cause stunting, discolouration and even tuber blacking. Plants also become more brittle with PLRV infection, which has been attributed to increased production of carbohydrate, this can also cause larger pest pressures due to the sweetening of the tissue.

The non-persistent viruses PVA (Potato Virus A) and PVY (Potato Virus Y) cause necrosis, mosaic symptoms and tuber defects in the crop.

Yield losses can be as high as ~80% for PLRV, and ~38%, ~25% for PVY and PVA respectively, this translates to field losses of around 10% overall. 

Ensure aphid species are identified in the crop to help with control and virus management. PLENUM should be used with Peach-Potato Aphids as no resistance has occurred, it also helps to disrupt feeding and therefore reduces virus transmission.

Work by Dr Alan Dewar has also shown good efficacy of PLENUM on Grain Aphid, which should be kept in mind with the heterozygous KDR resistance present in some of the Grain Aphid population.

HALLMARK ZEON will provide excellent control and return on investment for tackling susceptible species. ACTARA foliar application should be reserved for resistance situations when high levels of Peach-Potato Aphids are present. 

However, to ensure meaningful use of insecticides there is a tool on the AHDB Potatoes website (http://aphmon.fera.defra.gov.uk/) for tracking aphid flights, as well as your own yellow water traps. 

Insecticide Recommendations:

HALLMARK ZEON - Ware and Seed crops: Max single dose: 75 mls/ha. Max total dose: 300 mls/ha. Max number of sprays: 4. A 7 day interval must be observed. 

PLENUM - Ware crop: 0.3 kg/ha x 2 treatments. Seed crops: 0.3 kg/ha x 3 treatments. Last application 7 days before harvest. 

ACTARA - Seed crop: 80 g/ha foliar x 2 treatments or 1 x treatment if 100 g/ha applied in-furrow. Only one foliar treatment of ACTARA may be applied to seed crops if an in-furrow treatment of ACTARA was made at planting. Ware crop: 80 g/ha x 1 treatment. Last application 7 days before harvest. If ACTARA has been used as an in-furrow application at planting then the first foliar insecticide used in the crop must come from a different chemical group.