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Tuber turgor dictates bruising risk

Dry soil conditions should focus potato growers’ attenti


March 31, 2008
By Fruit & Vegetable

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Dry soil conditions should focus
potato growers’ attention on tuber quality as well as yield. They need
to be thinking about bruising and vascular browning – along with
irrigation scheduling – well in advance of harvest and not as a last
minute fix-it, advises Eric Anderson of Scottish Agronomy.

tireDry soil conditions should focus potato growers’ attention on tuber quality as well as yield. They need to be thinking about bruising and vascular browning – along with irrigation scheduling – well in advance of harvest and not as a last minute fix-it, advises Eric Anderson of Scottish Agronomy.

He believes growers could look to pre-treat drought stressed crops with a low-rate Reglone application early in the desiccation program, to reduce water loss from the leaves and give tubers the maximum opportunity to re-hydrate with available water, thus minimizing bruising.

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“We now know that drought stress over the month preceding desiccation can make tubers highly susceptible to bruising, rather than just the physical soil conditions as they are being lifted,” says Anderson.
pots
Trials in 2004 and 2005 confirmed the close relationship between dry soils at desiccation and bruising at harvest. But they also threw up an anomaly of excessive bruising levels even when soils at harvest were almost at field capacity, which Anderson attributes to a period of drought stress several weeks before lifting.

“Senescence of roots commences long before foliage. Clearly, once roots have started to senesce, there is much less opportunity for tubers to recover from any partial de-hydration. It doesn’t matter how much water you put on, if the tubers can not respond,” he adds.

“Small changes in tissue elasticity, arising from tuber turgor, have very large effects on bruising thresholds and must be addressed as early as possible,” advises Anderson. “But susceptibility to bruising also depends on the physiological and biochemical state of the tuber – which is now believed to be linked to the levels of cytokinins, abscisic acid and jasmonic acid, which all change with the development of the tuber.”

Anderson reports further trials in 2005 highlighted no increased incidence of vascular browning at the reduced rate Reglone applications, even in the driest conditions. “In fact, flailing has frequently resulted in the highest levels of vascular browning in trials, confirming tubers are pre-disposed to damage by very rapid haulm removal, rather than the method of desiccation.

“The problem is exacerbated when crops are under moisture stress at the time of desiccation, indicating that a pre-flail treatment with Reglone, five to seven days before topping, could help reduce any damage.”

He advocates crops suffering water stress should be sprayed with Reglone first thing in the morning, when tubers have had the previous night to re-hydrate.

He advises growers to avoid flail desiccation in wet soil conditions, when wheel damage could seriously compromise harvesting operations, and there is a real risk of increasing soft rot and blackleg problems. “A sprayer operating at 24 to 32 metres will significantly reduce soil damage, compared to a flail machine. And if Reglone is included with the blight spray program, there are no additional costs or spray passes.”


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