Potato brown fleck
Stephen Harper, Department of Primary Industries and Fisheries, Queensland.
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Introduction
In some Queensland regions internal potato tuber disorders are a major impediment to the production of quality fresh market potatoes (Solanum tuberosum). Brown fleck (BF) is the most significant of these tuber disorders. Symptoms of BF include an irregular blotchy brown discolouration of the inside (medulla) of the tuber (Figure 1). There are no obvious external symptoms. The disorder is more prevalent in larger tubers but in severe cases also occurs in small tubers. Consumers are put off by unsightly disorders, however a low incidence of BF does not affect the eating quality of the tuber.
Figure 1. Symptoms of brown fleck in Sebago potato tubers
Brown fleck is not caused by a pathogen (disease) and it is not transmitted from one tuber to the next. Plants grown from seed with BF symptoms will not necessarily produce potatoes with BF. Brown fleck is generally more common in crops where tubers bulk up rapidly, so is related to fluctuations in growing conditions, particularly those favouring rapid growth. The disorder is generally most prevalent at the apical or crown end, that is the opposite end to where the tuber is attached to the plant.
Around the world, disorders that result in tuber internal browning, have been variously referred to as brown fleck, internal heat necrosis, internal rust spot, chocolate spot, internal brown spot, mahogany browning, physiological necrosis and other names. In Australia brown fleck is the accepted name for internal brown lesions in the tuber.
Types and causes of brown fleck
DPI&F research has shown there are two types of BF, the first (early form) that can be initiated early in tuber development and the second (late form) that is usually initiated closer to maturity.
The early form can be initiated when tubers are about 50 mm in diameter, but can also be initiated at all growth stages through to tuber maturation. With this form of BF, phloem sieve cells in the tuber medulla are affected. These cells are responsible for transporting sugars (sucrose) produced in the leaves, to the tuber's storage (parenchyma) cells where the sucrose is converted into starch.
The late form of BF occurs mostly at around tuber maturation (about 110 to 120 days for Sebago). This form of BF affects healthy starch storage parenchyma cells, commencing with surface browning of mature amyloplasts (starch grains) and is favoured by warm soil conditions.
With both forms of BF the associated cell death results in browning reactions. There is a release of oxidising enzymes that are normally contained within the organelles of affected cells. With the release of these enzymes the disorder spreads throughout the affected cell and into healthy cells.
There is considerable variability in cultivar sensitivity to BF. In Australia Sebago, the dominant brushed table cultivar, is sensitive to the disorder, whilst other varieties including Atlantic, Desiree and Nadine are less sensitive.
Temperature
DPI&F research has shown that temperature is a major factor in the development and expansion of BF symptoms. In the Lockyer Valley, crops planted in March and harvested in July do not exhibit BF. However crops planted in June/July and harvested in October/November regularly exhibit BF, even when grown on the same soil type.
With the early form, the night time air temperature is an important factor determining the incidence of BF. It is likely that high night temperatures (mean temperature about 16 to 18°C), increase the amount and rate of sugar transport to the tubers, which increases symptom expression. High day temperatures do not increase incidence of this form of BF. Indeed, extremely high day temperatures are likely to reduce incidence since the photosynthetic capacity of the plant (the ability to manufacture sugars) is reduced under high temperatures (greater than about 30°C).
Soil temperature plays an important role in the development of both forms of BF. The late form of BF appears to occur when high soil temperatures favour the breakdown of the starch amyloplasts in the tuber storage (parenchyma) cells, resulting in cell browning. Soil temperatures greater than about 23°C cause a growth and expansion of symptoms of both forms of BF.
The role of foliage
Tubers from plants that remain green late in the crop often have more BF than tubers from vines which senesce prematurely. This is likely to induce the early form of BF. In contrast, the late form of BF symptoms appear to occur and increase when potatoes are held in the ground, with limited ground cover, after vine senescence and maturation.
A series of DPI&F field experiments demonstrated that foliage removal greatly reduced the incidence of BF. However, a major trade off with removing foliage is that potato yields can also be reduced. You need to consider this factor carefully when deciding wether to remove foliage to reduce BF incidence. There is potential to use foliage retardants to reduce potato growth at critical stages, but further research is required to evaluate forms, rates and timing of potential retardants.
Calcium and boron nutrition
In overseas research, application of calcium (Ca) and boron (B) has been associated with reduced incidence of internal tuber browning disorders, but the results have been highly variable and inconsistent. In Queensland trials, Ca and B application have not reduced BF incidence, so we do not recommend applying them to reduce BF incidence. However Ca and B should be applied if needed to meet crop requirements.
Irrigation
The incidence of BF is increased by excessive irrigation or rainfall. The effect is likely to be twofold, excess irrigation favours foliage growth and hence assimilate supply to the tubers, but also increases the osmotic pressure of the tuber cells.
Managing brown fleck
In regions where BF is a problem, grow the spring harvested crop 'a little on the hard side' to reduce the development of a vigorous canopy. This can be achieved by not excessively fertilising or irrigating.
Growers should monitor potato crops for the first incidence of BF. Cut tubers greater than about 5 cm diameter and look for evidence of necrotic cells. In the earliest incidence these lesions will be of about 1 mm in size (Figure 2). If early necrotic lesions are present, manage the crop to limit canopy growth.
Figure 2. Tuber section showing small brown lesions associated with the early form of BF
In conjunction with monitoring, use short term weather outlooks to identify impending weather conditions that would favour BF development. Conditions include warm night temperatures, mild day temperatures and impending rain.
Use tensiometers to determine soil moisture status, particularly around crop maturity when water use declines. This provides an objective basis for determining irrigation requirements and should prevent excessive late irrigation which is likely to favour development of BF symptoms.
Retain vegetative (weed) cover to shade the soil from direct sun. This maintains the soil at a lower temperature that is less favourable for expansion of BF symptoms and the development of the late form of BF.
If early incidence of BF is evident, foliage (top) growth is prolific, and impending weather conditions are favourable for BF development, foliage management could be considered. Bear in mind that although BF will be reduced yield could also be reduced.
Foliage development can be managed in various ways including; use of defoliants or retardants; restricting or withholding irrigation; and not applying excessive nutrients that favour foliage growth.
A major trade off with removing foliage is potential yield loss, but early removal (up to 14 weeks after planting) of up to 25% of foliage does not appear to affect final yield.
DPI&F information and services
- To access DPI&F's information and services, Queensland residents can contact the DPI&F Business Information Centre on 13 25 23 for the cost of a local call, from 8 am to 6 pm Monday to Friday (excluding public holidays). E-mail callweb@dpi.qld.gov.au. Non-Queensland residents phone (07) 3404 6999.
- Current national information on agricultural chemicals registered for use on all crops is available on the Infopest CD-ROM. Write to DPI&F, GPO Box 46, Brisbane, Qld 4001, E-mail infopest@dpi.qld.gov.au, visit the Infopest web page, or phone (07) 3239 3967 for further information.
Industry links
- Industry links is a page of links to various sites of interest to horticultural growers.
Information contained in this publication is provided as general advice only. For application to specific circumstances, professional advice should be sought. The Department of Primary Industries and Fisheries Queensland has taken all reasonable steps to ensure the information in this publication is accurate at the time of publication. Readers should ensure that they make appropriate inquiries to determine whether new information is available on the particular subject matter.
Last updated 28 May 2007
