Drowning of tobacco plants under poor drainage, and (right) wilting of tobacco grown in a vlei due to dying roots
Tobacco Research Board
In seasons with well distributed and adequate rainfall, tobacco grows well in all districts.
The crop takes up nitrogen at the correct times, grows fast and runs out of nitrogen shortly after topping.
It ripens evenly and produces good yields of excellent quality tobacco, provided it is reaped and cured correctly.
However, under continually moist conditions, as in the current season, tobacco growers face a lot of challenges.
One of the challenges is leaching, a condition where elements such as nitrogen and potassium are washed beyond the root zone making it impossible for the crop to access these nutrients, and deficiency symptoms develop.
Nitrogen plays a key role in controlling important quality parameters such as leaf colour and chemical attributes that lead to high leaf quality.
Leaching of nitrogen causes the tobacco crop to become pale and yellowed. This reduces the yield and grade of the crop and ultimately the price of the crop at the auction floors.
Potassium (K) deficiency leads to a condition known as potassium firing, which growers often mistake for a leaf disease. Potassium firing develops when excessive soil moisture slows down the diffusion of potassium to the roots.
Where leaching symptoms such as yellowing of leaves throughout the entire plant have been observed, growers are advised to immediately apply nitrogen in the form of ammonium nitrate, calcium nitrate or potassium nitrate.
It is not possible to be precise about the amount of nitrogen to be applied due to the fact that the extent of leaching in soil depends on the soil texture, the amount of moisture in the soil, permeability of the soil, amount of rainfall received, slope of the land, age of the crop, ridge orientation and the amount of nitrogen or potassium initially applied.
However, an application of 75kg of ammonium nitrate/ha (cup No.5/plant) or 150kg calcium nitrate/ha (cup No.8/plant) is usually sufficient.
Deep coarse-grained soils and late planted December crops may require more fertilizers (up to 300kg/ha).
Growers may choose the specific fertiliser to use, but it must be noted that ammonium nitrate is more persistent in the soil and is less prone to leaching than calcium nitrate.
In the case of potassium deficiencies, improvement of the drainage system, can lead to the problem self-correcting.
However, an additional application of potassium fertilisers is recommended in extreme cases when leaching has occurred and deficiency symptoms are severe.
In these cases growers can apply 100kg/ha (Cup No.5/plant) of sulphate of potash (SOP).
Mild symptoms of K deficiency (left) and the progression of K deficiency (right)
It is not advisable to apply muriate of potash (MOP or potassium chloride) because it also supplies chlorine which can reduce the quality of the cured leaf if it is supplied in excess.
Application of leaching adjustment fertilisers will not give positive results when there is poor drainage in the field.
As a preventative measure, in a year when heavy rains are predicted, it is important for growers to ensure that tobacco is grown on higher ridges to promote good drainage.
The furrows must be open at the ends to ensure movement of water out of the land.
This is important because even when nutrients are adequate, under prolonged water-logging conditions, plants can die due to a lack of oxygen in the root zone.
Tobacco grown on vleis is particularly prone to drowning.
The crop will show signs of false ripening first, all the leaves will turn yellow (banana), after which plants begin to wilt and die.
Where a grower has no choice but to produce tobacco in a vlei area, it is recommended that planting be done very early and that a short season variety such as KRK26R, T70 or T73 is used to facilitate complete harvesting of the crop before the main rains are received.
Leaf diseases such as angular leaf spot (Ps. syringae pv tabaci tox-) usually flourish under continually wet conditions.
It is, therefore, necessary to ensure that routine sprays of preventative Bion or Agrigold are applied for leaf disease control.
Highly fertilized tobacco crops easily succumb to foliar disease pressure, especially during continuously wet and humid conditions.
It is, therefore, recommended that growers optimize fertilization to attain high yields and quality tobacco.
Increased angular leaf disease incidence due to over-fertilisation and excessive rainfall.
In an overly wet season, plants will generally ripen faster and this means that the number of leaves reaped per cycle increases, necessitating additional resources to enable curing of the crop in a shorter space of time.
As a result, additional barn space over and above that for the planted hectarage must be available.
To assist in such cases, where barn space is still inadequate despite the preparations, the Tobacco Research Board has developed a low cost, easy to assemble, emergency plastic barn (Kutsaga 911 barn).
Growers can freely access the barn designs from the Engineering Section at Kutsaga.
When rains are incessant, it may be unavoidable to reap wet leaves, which predisposes them to mould development in the barn.
If leaves are reaped wet, they should be left to drip dry in the barn with the doors open before any temperature increase.
Temperatures should only be increased when leaves are dry, to avoid a ‘cooking effect’ that will result in poor cured leaf.
Good humidity and temperatures management is the key to barn rot control.
After curing, the leaf must be stored in clean, weatherproof, disinfected sheds with adequate ventilation.
Tobacco should not be over-conditioned and the relative humidity should be maintained at no more than 70 percent in untying and storage sheds.
Generally, for wet weather storage, slat packs are preferable to bulks and regular temperature checks are important.
Humidity can also be managed through the use of heaters, fans or fire pits to prevent the development of dark coloured tobacco during storage (precaution must be taken to avoid fires).
Baling of the leaf must ideally be done as close to the selling date as possible, and during storage, bales must consistently be monitored and inspected to ensure that the leaf moisture content does not exceed the recommended 17 percent.
Proper planning and timely conduct of routine activities will go a long way in reducing losses associated with high moisture conditions.
For more information, contact Kutsaga Research Station on telephone: (04) 575 289-94 or toll-free, 0800 4511 or Email: [email protected] or visit Kutsaga Research Station on Airport ring Road, Harare.