Plant Health Services Division, Tobacco Research Board Several insect pests, nematodes , bacteria and fungi , cause minor to severe damage to the roots, stem and leaves of tobacco plants at and just after planting.For example, in high infestation areas, cutworms can destroy the whole crop stand in a single night following transplanting of tobacco seedlings from the nursery to the lands.
Similarly, sore shin, a disease caused by a soil borne fungal pathogen Rhizoctonia spp. which occurs naturally in the soil can ravage transplanted seedlings , thereby resulting in gross loss to the grower .
A number of chemical pesticides are available for the control of pests and diseases in tobacco seedlings and transplants and these are discussed in the following sections.
Disease control prior to pulling out seedlings
Sore shin attacks young transplants in the first few weeks after transplanting when the stems are still tender. The disease is best managed by application of recommended preventive fungicides applied 48 hours before transplanting.
These fungicides are systemic and will protect the seedling for up to two weeks after drenching.
Therefore, if more seedlings are pulled from the treated seedbeds within two weeks from drenching, no further fungicide application is required.
However, if pulling is done later than two weeks after the first application, a second drench at half rate should be applied.
Nematode control at planting
Following the withdrawal of Methyl -bromide and Ethyl -di-bromide (EDB) as fumigants due to their adverse effects on the user and the environment, the Tobacco Research Board (TRB) embarked on research to evaluate alternatives to the toxic fumigants. Thus, in addition to using nematode resistant varieties, products such as Fluopyram (Velum 400 SC), Fenamiphos EC and Oxamyl are registered fornematode control.
Pre -planting insecticides should be applied in the planting hole to control soil-borne pests such as white grubs, false wireworms and cutworms as well as foliar pests such as aphids.
For cutworm control, the insecticide should be applied such that it wets the stem and the soil surrounding the base of each seedling.It is important to plant with preventive aphicides to ensure plants are protected against aphids, which besides, causing direct damage through sucking plant nutrients, are also vectors of diseases such as Potato Virus Y (PVY) and bushy top.
When any of the aphicides in Table 3 is applied at the time of transplanting, this ensures protection for 6-8 weeks after planting.It is also wise to plant early so as to reduce the impact of some pests of economic importance.
For example, tobacco planted from September into mid-October is able to evade the seasonal build-up of the tobacco aphid that usually occur in late November and reaches a peak in mid -December to January.
Growers are encouraged to adopt these pests and diseases preventive measures to ensure healthy crop growth, free from pests and diseases.
◆ For more information, refer to the Tobacco Research Board Flue – Cured Recommendation or contact Kutsaga Research Station’s Plant Health Services Division on telephone (04) 575 289-94 or toll-free, 0800 4511 or Email: [email protected] or visit Kutsaga Research Station, Airport Ring Road, Harare.