- Conditions which increase the risk of internal rust spot
- Should additional calcium be applied to mitigate internal rust spot?
Vascular browning and stem end necrosis
- Often associated with application of diquat in dry soil conditions, vascular browning and stem end necrosis can result from physiological shock as a result of rapid destruction of a vigorous, green canopy, particularly in dry soil conditions.
- Symptoms may occur as a result of chemical or mechanical haulm destruction and cultivars differ in susceptibility.
- Destruction of an immature canopy is difficult and may require a combination of chemical and mechanical treatments but soil moisture should be checked before treatment begins.
- If soil from the centre of the ridge, 5 cm below the lowest tuber, is too dry to form into a ball, there is a risk of poor tuber quality if an active canopy is killed rapidly.
Internal rust spot
- This is a physiological disorder characterised by brown flecks or spots distributed unevenly through the tuber.
- Internal rust spot (IRS) is associated with calcium deficiency which results in loss of membrane integrity and cell necrosis; other internal necrotic disorders may also result from calcium deficiency.
- Calcium has little mobility in phloem or from cell to cell and concentrations in tubers are very low.
- High rates of potassium application may reduce total uptake of calcium by potatoes but there appears to be little influence on the concentration of calcium in tubers.
- Calcium can be transported out of tubers more easily than in to tubers and there may be loss of calcium from tubers under dry conditions, when water may be lost from tubers to foliage. Application of a calcium product to foliage may reduce stress and help tuber calcium levels to be maintained.
- Large tubers and those with a high specific gravity may have a higher incidence of IRS.
- IRS may increase during storage, particularly at higher temperatures.
- Cultivars differ in susceptibility to IRS, however this does not appear to be related to the calcium content of the tubers.
Conditions which increase the risk of internal rust spot
- The disorder is induced by growing conditions which result in uneven tuber growth, i.e.:
- restricted or irregular supply of water
- high temperature
- temperature / moisture fluctuations
- inadequate calcium supply
Should additional calcium be applied to mitigate internal rust spot?
- Application of calcium to soil produces variable results and very high levels (e.g. 4 t calcium sulphate ha-1) may be required to have any influence on internal necrotic disorders.
- There is no evidence that foliar applied calcium directly influences tuber calcium.
- There may be an indirect effect due to alleviation of stress preventing loss of calcium from tubers.
- Reductions in internal rust spot may be small but there may also be reductions in other internal necrotic disorders.
- Limited evidence suggests that there may some beneficial effects from foliar application of calcium products.
See the Calcium page for more information on application of calcium.
- This is a physiological condition which may develop into hollow heart.
- There are no external symptoms but tubers may contain areas of brown, necrotic cells, surrounded by cells from which starch has been depleted.
- Hollow heart may be initiated by stress conditions during early stages of tuber growth, including low temperatures, very wet soil conditions or drought stress.
- If the stress is of short duration and occurs before cell division has finished, tubers may seem to “recover” as necrotic cells are dispersed during cell division.
- This is a physiological condition characterised by a hollow in the centre of a tuber, often surrounded by a corky layer. There are no external symptoms.
- Hollow heart is usually associated with large tubers which have had fast rates of growth.
- Hollow heart may also occur in small tubers which have experienced stress and may be initiated early or late in the season.
- Hollow heart development may or may not be preceded by brown centre.
- Cultivars differ in susceptibility to hollow heart.
- Hollow heart occurs most frequently in situations with uneven water supply, where drought stress alternates with rapid re-growth. Water management is therefore the most effective means of reducing the risk.
- The problem may be exacerbated by potassium deficiency (potassium is important for tuber water relations) and calcium and carbohydrates may be lost from tubers under stress conditions.
Jelly end rot
- This is caused by stress and is usually a problem at the stem end of the tuber.
- Affected tubers are usually pointed towards the stem end.
- Compared with the rest of the tuber, affected tissues have:
- lower specific gravity
- lower starch concentration
- higher sugar concentration
- Processed tubers have poor fry quality with uneven colouration.
- If the problem is severe, the tuber end can shrivel and collapse into a jelly like mass of tissue but there can be distinct boundary between healthy and affected tissue.
- Long tubers, such as “Russet Burbank” are particularly susceptible.
- If severe stress occurs during later growth, the problem may also occur in oval or round shaped tubers.
- Avoid drought stress.
- If there is a risk of high temperature stress becoming a regular problem, plant susceptible cultivars earlier, so that there is a greater likelihood of early tuber development occurring at a lower temperature.
- Examine tubers prior to harvest – if there is a potential problem allow tubers to mature in the field to reduced the risk of progression of the condition.
- Immature tubers are more prone to development of the condition if stored at temperatures below 7 oC than at higher temperatures.
- What is this?
- A physiological condition causing potatoes to rapidly turn brown after cutting and exposure to air.
- How important is it?
- A problem in “Pentland Dell”.
- An occasional problem in other crops but can be serious when it does occur.
- What can be done?
- The food processing industry can use some techniques to mitigate against it.
- Crops with a serious problem can be rejected – “prevention is better than cure”!
Enzymic compared with non-enzymic browning
- What is the difference?
- Enzymic browning occurs as a result of biochemical reactions catalysed by enzymes:
- discolouration after cutting
- discolouration after bruising
- these are very similar reactions
- Non-enzymic browning does not depend upon the action of enzymes:
- the reaction between reducing sugars and amino acids at high temperature is non-enzymic browning
- this is the Maillard reaction which results in dark colour chips and crisps
Reducing the risk of enzymic browning
- A potentially serious problem with limited research on prevention / avoidance.
- Cultivars differ in susceptibility.
- Stress (including nutrient deficiencies, very high or low temperatures, excess water), diseases and insect attacks may increase the risk.
- If this condition is of concern, tests should be included in crop monitoring.
- If internal browning is found, tubers should be stored at a low temperature and for as short a time as possible.
Notes on physiological disorders
Enzymic browning 2016 (Development of enzymic browning after cutting; comparison with non-enzymic browning)
Weather and physiological disorders 2012 (Effects of extreme weather conditions)
Jelly end rot 2012 (Development of jelly end rot)
Hollow heart 2006 (Development of brown centre and hollow heart)
Haulm destruction 2005 (Includes physiological vascular browning and stem end necrosis; N.B. information on desiccants has been superseded)
Internal rust spot 2000 (Stress can result in loss of calcium from tubers and increased risk of internal rust spot)
Internal rust spot 1998 (Relationship between calcium and internal rust spot)