Production of tobacco seedlings using float trays is an excellent and proven way of producing even, well-hardened seedlings with a vigorous root system that enables seedlings to take off faster and withstand extended periods without water in the field. In this current season, a large number of growers have adopted the floating tray system for tobacco seedling production and are producing good seedlings using this system. However, there are a few areas as outlined below that growers should pay attention to so that they consistently produce very good seedlings.
1. Management of Fungus gnat
The proper construction of ponds minimises algae. Algae build up in the trays and on the sides of the float trays indicates poor bed construction. If trays do not fit snugly
in the bed, algae will grow in the areas exposed to sunlight. While algae in itself is not necessarily a huge problem, its growth promotes fungus gnat breeding. Chemical applications (drenching) of quaternary ammonium compounds such as Sporekill 12% or Pentakill 20%) can be used to control established infestations of algae to deprive Fungus gnat adults of a source of food.
However, once established, high populations of the flies can decimate young (just germinated) seedlings and leave a seedbed utterly devoid of seedlings. The use of yellow sticky traps (Kutsaga Gnatbuster®) is an effective monitoring tool and should be used for the early detection of an infestation. If detected, fungus gnat can be controlled by drenching with Cyromazine (30 g/100 L water) or Imidacloprid + β cyfluthrin (60 ml/100 L water) starting at 5 weeks after sowing and should be repeated at 10 to 14 day intervals.
2. Management of Salt Injury
During the hot months of August to September, seedlings are prone to salt injury which manifests as cupped leaves, brown colouring of the hearts and eventual death of the affected plants. High salinity occurs when there is an accumulation of salts on the surface of the growing medium as a result of excessive evaporation. The water that evaporates from the surface of the medium is replaced by floatbed water resulting in the continuous movement of water and salts through the cells and a consequent accumulation of salts at the surface.
Salt injury can be prevented by using floating row covers or a grass mulch to cover the seedlings and reduce evapo-transpiration. In cases where this damage has already set in, watering from the top using a can or a hose fitted with a fine rose can alleviate the symptoms as the salts are washed down into the pond water.
Should the grower have any queries, please do not hesitate to contact the Tobacco Research Board’s Crop Productivity Services Division or Plant Health Services on (04) 575 289/94 or toll-free, 08004511 or Email: tobres@kutsaga.co.zw or visit Kutsaga Research Station.
By Rhoda Mavuka and Zimazile Jazi, TRB