10 April 1998

Anti-rust spot calcium treatment proves worth

Severe internal rust spot

problems in many potato

crops last year have focused

attention on preventative

treatments this year. We take

a closer look at one option

A SHARP rise in internal rust spot in potatoes last year was due to calcium deficiency brought on by fluctuations in soil temperature and moisture.

That is the view of Gidon Bahiri of nutrient supplier Omex. "Internal rust spot is caused by stress initially. This gives rise to a calcium deficiency in the plant, which then manifests itself in the characteristic brown ring inside the tuber. In the absence of stress, the symptoms do not appear," he says.

Until 1997 it was difficult to know if the condition was becoming more prevalent or just being noticed more. "Growing for quality markets made farmers more aware of it," he suggests. "But last years unusual weather – a dry period followed by heavy rain – gave a tenfold increase in its incidence."

Stress during early tuber growth is the most damaging, he warns. "If there is calcium deficiency when the tuber is pea-sized, rust spot will develop. Once the symptoms are visible, it is too late to do anything about it."

Light land crops subject to greater temperature and moisture fluctuations causing stress are most susceptible.

Mr Bahiris advice is to get calcium on to the crop early. "But calcium is not very mobile, so it is difficult to get into the plant. Foliar applications are better than soil-applied calcium, as it gets locked up with phosphate in the soil."

Trials with Omex product Calmax show it can cut internal rust spot by 20-30%. Added benefits are a reduction in hollow heart and improved long-term storage. Mr Bahiri suggests two approaches to application timing. "Either make three applications, starting at early tuber initiation and repeating it every 2-3 weeks until the tuber is 3in across. Alternatively, use sap testing to give an indication of calcium levels and see if they are falling. If leaf levels are dropping, the same will be happening in the tuber. Last year, levels were literally plummeting."

Calmax can be mixed with blight sprays, making application straightforward. Three applications of the 24% product provide 300gCa/ha (4.3oz/acre). However, its low mobility means only a proportion of this ends up in tubers.

Including boron in Calmax aids calcium uptake. "The two nutrients seem to carry each other down into the tuber," stresses Mr Bahiri.

No guidelines or thresholds for tuber calcium levels exist yet. "But you cant overdose with calcium, as it is so difficult to get into the plant."

The cost for three applications is £21/ha (£8/acre). "There is some yield benefit, but the main advantage from applying foliar calcium is safeguarding quality. Potatoes with internal rust spot are subject to rejections and can be worthless."

Low calcium or spraing?

Calcium deficiency is easy to confuse with spraing. A virus test will differentiate between the two. And some varieties are worse than others – both Maris Piper and Russett Burbank seem particularly prone to it.