Arable Updates

Key points

• This three year programme of work focuses on nitrogen fertiliser decisions for maize, potato and leafy green crops. This Update summarises the potato component of the project.
• The Quick Test Mass Balance approach is an effective method for informing in-season N management decisions for potato growers.
• Nitrate Quick Test strips can be used to estimate mineral N levels in the soil at any time during the crop’s growth. They may be used as a cost effective substitute for the mineral N test, particularly for top soil samples (0–30 cm depth), where nitrate-N is the predominant form of mineral N.
• Quick Test estimates of soil mineral N levels can be used for fertiliser mass balance calculations to inform in-season N management decisions.
• N Fert_SD = [Crop N_Total – Crop N_SD] – Soil Min N_SD – Soil Org N_mineralised. Information about the potential crop demand and the soil nitrogen supply is required.
• The mass balance calculation for a side-dressing decision is:
• The mass balance calculation can either verify the fertiliser plans for the crop are correct, or support less fertiliser being applied. It can also indicate more fertiliser should be applied to reach the potential crop yield.

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Key points

• Year two of this three year SFF project focused on soil structure and rotation history for 15 sites in Canterbury. There were four-plot trials at each site (Russet Burbank and Innovator, treated with formalin and untreated).
• The major influence on yield was soil quality, and seedsoil borne disease had little impact.
• The crop history score x soil structural condition score factor explained 39% of the yield variation for Innovator and 52% for Russet Burbank. If soil quality is poor then growers should consider growing Innovator in preference to Russet Burbank.
• There was a good correlation between yield and a 10 year crop history score, and between yield and a oneoff soil structural condition score, showing that these two independent methods could be useful for gauging paddock suitability for growing potatoes.
• More grass in a ten year history improved soil resilience and enhanced rooting hospitality for potatoes, thus enabling the crop to access more resources. For Russet Burbank, this equated to an average 3.5 t/ha lift in yield for every year in the previous ten year history a field was in grass.
• Formalin dipping did not significantly control these diseases in the glasshouse or the field.
• Seed could have transferred Rhizoctonia solani, causing stem canker, and Spongospora subterranea, causing root galls, to the field, as all glasshouse plants were infected with these diseases.
• Stem canker and root gall incidence and severity was greater from an ex-potato paddock history compared with no potato history, but yield was unaffected.
• Disease severity was higher in fields with predominantly grass histories, compared with mainly cropping histories. However, gross yield was greater from ex-grass (86 t/ha) than from ex-crop fields (75 t/ha).
  

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The severity of seed and soil-borne diseases varies by site and season. This update provides further information from one high risk site in South Canterbury.

Key points

• The lack of efficacy of fungicides is a major concern, particularly if they are reducing yields. There is a significant cost associated with applying fungicides and if, at a number of sites in New Zealand, they are providing no control and potentially reducing yield then the reason needs to be understood.
• No fungicide treatment increased yield in this trial, or in the three trials in the previous two seasons.
• To date this project has not identified any new control strategies which are more effective than commonly used treatments.
  

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Key Points:
• A replicated trial was set up in a commercial crop at Levels, South Canterbury with potato cultivar Innovator, planted on 12 October 2015. The trial site was four years out of potatoes.
• A number of diseases were found in the sampled plants and tubers including Spongospora root galling and tuber powdery scab; Rhizoctonia stem canker and tuber black scurf; Sclerotinia white mould on stems, black leg on stems, and common scab on tubers.
• Rhizoctonia stem canker and Spongospora diseases predominated, while the other diseases were at very low incidence levels.
• Nebijin® reduced the severity of powdery scab on tubers at both assessment timings and this reduction was statistically significant when compared to the nil treatment. None of the other treatments affected any of the diseases observed in the trial.
• There were no statistically significant differences between the treatments for unmarketable or marketable yields. Overall mean yield of marketable tubers was equivalent to 82.8 t/ha.

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Key points

• Field trials were established in Pukekohe, Manawatu and Canterbury to develop regionally focused pest management strategies, initially focussing on tomato potato psyllid (TPP) and zebra chip disease.
• The accumulated degree days trigger used in treatment 2, has not worked in Canterbury for two years in a row. This is in contrast to trials in the North Island where these treatments work well. This season, a refined trigger will be tested in Canterbury.
• Zebra chip disease incidence can be highly variable in a crop. It is determined by TPP distribution in a field and how many psyllids carry the bacterium that causes the disease.
• Spray timings seem to be quite important to manage zebra chip disease.

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