insects, etc. once the bacteria have entered the plant, they occur intercellularly and stimulate surrounding tissue to proliferate due to cell transformation. agrobacterium performs this control by inserting the plasmid t-dna into the plant's genome. see above for more details about the process of plasmid dna insertion into the host genome. excess growth of the plant tissue leads to gall formation on the stem and roots. these tumors exert significant pressure on the surrounding plant tissue, which causes this tissue to become crushed and/or distorted. the crushed vessels lead to reduced water flow in the xylem. young tumors are soft and therefore vulnerable to secondary invasion by insects and saprophytic microorganisms. this secondary invasion causes the breakdown of the peripheral cell layers as well as tumor discoloration due to decay. breakdown of the soft tissue leads to release of the agrobacterium tumefaciens into the soil allowing it to restart the disease process with a new host plant. disease management crown gall disease caused by agrobacterium tumefaciens can be controlled by using various methods. the best way to control this disease is to take preventative measures, such as sterilizing pruning tools so as to avoid infecting new plants. performing mandatory inspections of nursery stock and rejecting infected plants as well as not planting susceptible plants in infected fields are also valuable practices. avoiding wounding the crowns/roots of the plants during cultivation is important for preventing disease. in horticultural techniques in which multiple plants are joined to grow as one, such as budding and grafting these techniques lead to plant wounds. wounds are the primary location of bacterial entry into the host plant. therefore, it is advisable to perform these techniques during times of the year when agrobacteria are not active. control of root-chewing insects is also helpful to reduce levels of infection, since these insects cause wounds (aka bacterial entryways) in the plant roots. it is recommended that infected plant material be burned rather than placed in a compost pile due to the bacteria's ability to live in the soil for many years. biological control methods are also utilized in managing this disease. during the 1970s and 1980s, a common practice for treating germinated seeds, seedlings, and rootstock was to soak them in a suspension of k84. k84 is composed of a. radiobacter, which is a species related to a. tumefaciens but is not pathogenic. k84 produces a bacteriocin (agrocin 84) which is an antibiotic specific against related bacteria, including a. tumefaciens. this method, which was successful at controlling the disease on a commercial scale, had the risk of k84 transferring its resistance gene to the pathogenic agrobacteria. thus, in the 1990s, the use of a genetically engineering strain of k84, known as k-1026, was created. this strain is just as successful in controlling crown gall as k84 without the caveat of resistance gene transfer. environment crown gall of sunflower caused by a. tumefaciens host, environment, and pathogen are extremely important concepts in regards to plant pathology. agrobacteria have the widest host range of any plant pathogen, so the main factor to take into consideration in the case of crown gall is environment. there are various conditions and factors that make for a conducive environment for a. tumefaciens when infecting its various hosts. the bacterium can't penetrate the host plant without an entry point such as a wound. factors leading to wounds in plants include cultural practices, grafting, freezing injury, growth cracks, soil insects, and other animals in the environment causing damage to the plant. consequently, in exceptionally harsh winters, it is common to have an increased incidence of crown gall due to the weather-related damage. along with this, there are methods of mediating infection of the host plant. for example, nematodes can act as a vector to introduce agrobacterium into plant roots. more specifically, the root parasitic nematodes damage the plant cell, creating a wound for the bacteria to enter through. finally, temperature is a factor when considering a. tumefaciens infection. the optimal temperature for crown gall formation due to this bacterium is 22 °c (72 °f) because of the thermosensitivity of t-dna transfer. tumor formation is significantly reduced at higher temperature conditions.