The continued planting of tomato plants in the same field is a common practice many people practice, but this can lead to some serious consequences in the yield from such products. The presence of the pathogens Fusarium oxysporum f.sp lycopersici (Fusarium wilt) and Ralsotnia solanacearum (Bacterial wilt) can cause severe problems in the production of not just tomato, but also potato, eggplants, and pepper crops.
In order to properly understand how to deal with these problems, one must understand how each pathogen operates, how it reproduces, how it develops, and how to identify the disease itself. These steps can be broken down into a few categories. These are disease symptoms, conditions for disease development, and pathogen description. All these will lead to the control methods for the pathogen.
Fusarium wilts disease symptoms include a yellowing of the lower foliage. The yellowing will progress up the plant and the lower leaves will dry out and turn brown. It is essential to note the yellowing of the leaves, as Bacterial wilt has similar symptoms and will look much the same. The plant will begin to wilt at the top of the plant during the day, but will recover at night. This wilt will become progressively worse, until the plant becomes permanently wilted. Vascular browning will occur far up the stem into the large petiols of the plant.
The wilt itself is a form of a fungus. It spreads via three mechanisms. The fungus spreads by the production of conidia. The fungus produces microconidia, macroconidia, and chlamydospores. These allow the plant several methods to inhabit a host plant and spread further. The microconidia are produced for rapid spread once actually in the host plant. The macroconidia is a larger conidia that is used for overwintering so that the fungus will survive while the host plant is dead. The chlamydospores is a combination of both microconidia and macroconidia. This large fungal spore allows the pathogen to actually survive several seasons without a host plant. This makes eradicating the pathogen very difficult.
The wilt itself favors warm weather. This will increase the disease development. This disease is most prevalent in acidic, sandy soils. Since a majority of North America's production of tomato crops comes from Florida and California, this poses a major problem. The pathogen is soil borne in nature and persists many years in the soil without a host. There are three known races of the fungus.
While rather difficult to eliminate, there are several methods that can be done to reduce the impact of the fungus on a crop. One very common method is to use a resistant cultivar. There are commercially available crops that are Race 1 and Race 1,2 resistant. Another fairly easy way to control the disease is to change the pH of the soil A pH of 6.5-7.0 will help contain the fungus. One cultural practice that can be used to contain the fungus and stop spread to other fields is rather easy. Cleaning of equipment when moving from field to field will help stop the spread of the fungal spores. The use of a 5-7 year crop rotation, with one of those crop rotations being a flooded rice field, will greatly reduce crop loss.
Bacterial wilt is a less common common problem, but can become devastating. The disease if very devastating to the plant, and can cause entire plants to die in a short amount of time.
The main plants that are affected by this are tomato, tobacco, potato, pepper and eggplant.
The bacteria is a gram negative rod, which means that the bacteria is resistant to the dying methods used when identifying the pathogen For more information on this, please see http://www.newton.dep.anl.gov/askasci/mole00/mole00791.htm
The bacteria occurs in scattered plants or groups of plants. The initial symptoms are wilt of the lower leaves or the upper leaves of seedlings. This is followed by a sudden permanent wilting of the entire plant without yellowing. Vascular browning occurs, and sometimes there is cortial decay found near the soil line. Bacterial streaming can occur from the vascular elements of the host. This can be seen by taking a cross section of a lower stem and suspending it in water. If a stream of bacteria comes out, then bacterial wilt is occurring.
The bacteria develops readily in the tomato, tobacco, eggplant, and potato plants. It is more damaging, though, in the pepper plant. It is a soil borne pathogen, and is able to survive for a long period of time in the soil. The bacterium gains entry through natural root wounds, insect or nematode wound, and cultivation wounds. A high temperature and high soil moisture regimen favors disease development.
The control methods for bacterial wilt generally begins (and mostly ends) with the production of the transplants. Using pathogen free seedbeds is paramount, as this will provide a disease free transplant. All planting beds should be fumigated and planting mediums should be pasteurized. Special care should be taken if pasteurizing medium though, as nitrogen toxicity can become of an issue due to the destruction of beneficial bacteria which provide nitrogen to the plant. Of limited value is the ability to rotate your crop with a non-susceptible crop. A major cultural practice that should be followed is to avoid cultivation that damages roots. If producing field grown crops, include a flooded rice patty in a crop rotation schedule.
Hopefully, these methods can be used in order to both identify problems in your fields as well as prevent any problems in the future.
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