Crops
Protection Methods
Presently, there is no single method for complete control of Phytophthora nicotianae. Successful control depends on the implementation of several partially effective complementary control methods.
- During cultivation
Nursery irrigation should be limited to meet needs and maximum ventilation of the shelters should be implemented. It is also critical to soak the entire substrate with an anti-oomycete fungicide solution. In the floating seedbeds, fungicides may be applied in the nutrient solution. The rate used should be low enough to avoid chemical injuries.
If symptoms occur after transplanting, a fungicide solution should be applied around the collar of the plants that soaks the soil thoroughly at that level. Foliar infections are usually well controlled by anti-mildew treatments applied on the leaves.
No fungicides are currently registered for this use in France.
Infected seedlings should be removed quickly from the nurseries. If they are planted, they will help to spread Phytophthora nicotianae and contaminate healthy soils. Non-visible infections are possible, they cannot be eliminated by the sorting of the seedlings.
At the end of the season, it is very important that tobacco stalks and root systems are removed from the field, in order to prevent increasing the population of pathogen chlamydospores in the soil.
- Next crop
In the nursery it is essential to use a clean and disinfected substrate. It should not be mixed with sand or soil collected from a field, as it may be contaminated. It is not recommended to lay the plugs on the soil, especially if it has not been disinfected. It is best to store them on the tables or on a plastic film. The seed layer should be disinfected. Disinfection can be done by spraying with a preventive fungicide solution propamocarb (e-phy). Control of black shank in floating seedbeds can be normally accomplished if the trays are protected from possible contamination by dust, mud, or rain/mud splashing when left in a field. Avoid use of pond or canal water, which is often contaminated, to fill the beds. For more details, please see the fact sheet on Protection measures to be adopted in the nursery.
Equipment used for soil cultivation in infested fields should be well cleaned before use in other disease free fields. This principle also applies to the tractor wheels. Thorough rinsing with water and followed by disinfection of the equipment is usually sufficient to eliminate the fungus contaminated adhering soil.
Crop rotation is widely recommended. In the case of this pathogen they are easily implemented because the disease is very specific to tobacco and does not normally infect any other crops. To be effective, the rotations should be as long as possible with a minimum of two years between tobacco crops.
Fertilisation and irrigation should be well managed to avoid excessive nitrogen. Control of root-knot nematodes can reduce the incidence of black shank (see the recommendations for protection against nematodes in the section on Root nematodes).
There are varieties with different levels of resistance to black shank. The resistance originates either from the cultivars of Nicotiana tabacum or other Nicotiana species.
Two wild species, Nicotiana longiflora and Nicotiana plumbaginiflora, have a dominant monogenic resistance that gives them a complete level of resistance to race 0 of Phytophthora nicotianae. This highly specific resistance was transferred to Burley (L8), Virginia, and flue-cured tobacco. Its use has only a limited value given the fairly widespread presence in fields of other races, especially race 1 that are able to overcome this resistance. Race 0 and race 1 are very common. Two other races have been described: race 2 in South Africa and race 3 in Connecticut, USA.
A number of selected varieties in the USA and Cuba show a partial resistance to different existing races. Resistance originates either from a cigar tobacco cultivar (involving a polygenic recessive resistance) or from Beinhart 1000-1, which shows a partially dominant oligogenic resistance. Kentucky 17 genotype seems to have this type of resistance.
Resistant varieties are used in several countries, especially in the USA. Their use is most common in fields with history of black shank.
*The number of pesticides available for a specific use is constantly evolving but the author has nonetheless chosen to include the names of some registered active ingredients at the time of writing these pages. He will try to update this list as and when withdrawals and new registrations are made. When choosing your crop protection method, please refer to legislation prevailing in your country. This remark is also valid for all organic products based on micro-organisms or natural substances.
U.S. situation
In the USA an integrated approach is implemented to manage black shank that includes the following measures:
Crop Rotation - Rotation is the foundation of any black shank management program because the fungus attacks only tobacco. Leaving the field out of tobacco for one or more years will reduce, but not eliminate this fungus. Any crop can be grown between tobacco crops to reduce the population level of the pathogen. The longer the rotation the better the result is.
Drainage - Improving drainage in poorly drained areas reduces black shank losses by making the soil environment less favorable for infection.
Sanitation - Elimination of the host by destroying stalks and roots immediately following harvest will help reduce populations of the fungus and nematodes, resulting in less damage in the next crop.
Nematode Control - Control of root-knot nematodes helps to ensure the effectiveness of resistant varieties and fungicides. In fields infested with both the black shank fungus and root-knot nematodes, the effectiveness of black shank resistant varieties and fungicides is reduced if the nematodes are not controlled.
Resistant Varieties - Varieties possessing various levels of resistance to black shank are available. There are two sources of resistance incorporated in tobacco varieties planted in the USA. The FL 301 resistance has been the predominant form of resistance for many years. It is effective to varying degrees against all black shank races. A more recently incorporated form of resistance imparts complete resistance (immunity) to race 0 of the pathogen but is susceptible to race 1. This complete resistance is controlled by a single gene (ph). However, varieties with the ph gene may vary in their resistance to race 1, depending on how much FL 301 resistance is in their heritage. Most new varieties released over the past 10 years have the ph gene. Use of a variety with the ph gene for two or more tobacco crops results in the black shank population changing progressively, or in some cases rapidly, from race 0 to race 1. When this occurs, the varieties with the ph gene will appear to have little resistance, and fungicide applications are needed.
Chemical Control - Several soil-applied chemicals are labeled for black shank control. Included are the multi-purpose fumigants Telone C-17, Chlor-O-Pic 100 at the same rates used to control Bacterial wilt. However, the systemic fungicides mefenoxam or metalaxil are the most effective materials against black shank. In over 30 field trials conducted for several years it appeared that applications of a fungicide early in the season (i.e. 1st cultivation) are more beneficial than later applications (at layby). See label for exact rate of application.
(Mina Mila - North Carolina State University)