Agrobacterium tumefaciens is a rod shaped gram negative motile bacteria that lives in the soil. It has flagella that allow it to swim through moisture films in soil and on plants. On its own its harmless, but when combined with the Ti plasmid it’s a nightmare, and causes large ugly cankers called crown galls, and can substantially reduce production and cause plant death. It is very cosmopolitan and can affect more than 60 different plant families. It is a major threat to the floriculture industry.
How does it work?
Agrobacterium with the Ti plasmid, swims through moisture films and is attracted to plant exudes in wounded plant tissue and plant roots. It attaches to a wounded cell and inserts the Ti plasmid into the cell. The Ti DNA combines with the plant DNA to induce tumour formation by switching on the production of cytokinins, indoleacetic acid, and novel plant metabolites called opines and agrocinopines. These feed the bacteria. The plant diverts much of its energy into producing galls, becoming weaker. This affects production and quality and the plant’s natural resistance to insect and disease pressure.
How to prevent Agrobacterium
Growers should source clean plant material from responsible breeders and propagators and plant into clean soil/media. Some propagators have an ELITE accreditation, declaring their plant material Agrobacterium free. Always check the bud-wood before deciding on a variety. Do not plant into soil that has been previously growing infected plants. In hydroponics, plant into clean new media. Sterilise any equipment that will be re-used (driplines, supports, poles, posts etc). Do not allow movement of people and tools between infected and clean greenhouses. Plants are wounded during normal daily cultural practices: grafting, planting, bending, pruning, and harvesting.
Staff awareness must be developed at all levels and strict hygiene practices put in place. Extensive equipment disinfection and hand cleaning should be automatic. Secateurs should be sharp and clean and dipped into disinfectant after every 20-30 stems harvested. Spray hoses, gumboots and aprons should be disinfected between houses. Re-cycle water should be heat treated, UV treated or passed through an ultra-filtration machine. System efficiency should be tested regularly with a standard plate count test. Breeders and propagators should cover the soils in their greenhouses to prevent soil contamination. Disinfectant footbaths and hand washing at the entrances of the greenhouses are easy to implement and help control the spread.
Living with the problem
Many farms in Kenya are infected with agrobacterium. Bacterial infections in plants are very difficult, if not impossible to treat. Farmers should reduce plant wounding as much as possible. Posts at the end of the paths prevent spray hoses passing over the plants. Always use sharp clean secateurs with straight cuts for harvesting. Trim the bending so that the plant does not get bashed every time someone walks past. All equipment should be cleaned and disinfected on a daily basis, and when moving from one greenhouse to another.
Keep plant stress to a minimum, larger galls tend to be on stressed plants or very rapidly growing plants. Keep the leaf area as large as possible, with a good canopy. Grow plants out of flush. Vigorous new growth can induce a new cycle of gall production. Keep insect and disease pressure to a minimum. Do not overspray the plants, each pesticide spray brings plant stress and extra opportunity for infection. Regularly test soil/media/water and feed the plants with an optimum nutrition. Nutrient in-balance, high EC, high and low pH and excess nitrogen can all induce extra gall production. Infected plant matter and galls should be taken from the greenhouses in bags and destroyed, and not used for composting. Proper climate control of greenhouses is most important. Hot dry greenhouses = stressed plants = lots of galls
Duponchella moths, fungus gnats and sciarids create plant wounds at or under the soil level which can very rapidly spread agrobacterium from plant to plant. They can also breed inside the old galls. These are easily controlled with various sticky traps and pheromones.
Agrobacterium initially starts as small swellings on wounded plant parts. Young tumors resemble plant callous tissue and are soft, creamy white and rounded. As they grow and age the shape becomes irregular and the tissue turns dark brown or black. The galls may be connected to the plant by a small piece of tissue and easily removed, or integrated into the stems/roots as a swelling that is not distinctly separate and can only be removed by pruning the plant. Painting the tumors with various concoctions can help with the ageing process and removal of tumors:- oily films, fresh cow manure, copper oxychloride, disinfectants e.g. Dettol have all been used with differing success.
However the practice is very labor intensive. Many farms cut the galls out by pruning. This is not recommended – it creates fresh wounds, moves the bacterium around the greenhouse, and quite quickly you can end up with a stressed plant with no canopy that is more susceptible to the infection.
Seeds and transplants can be treated biologically with the non-pathogenic Agrobacterium radiobacter K84. This is contains a plasmid called pAgK84 which codes for agrocin 84 production. The bacteria competes for the agrobacterium’s food source and also kills the pathogenic bacteria with agrocin 84. Sadly this treatment is not available in Kenya at the moment.
Treatment with antibiotics is not recommended. Extensive non-purpose use of antibiotics can cause a resistance build up in human pathogens, which means that the antibiotics won’t work when required.
Agrobacterium is essentially a soil bacteria and as such it cannot withstand high temperatures. Important rootstock and budwood can be professionally cleaned by a heat cycling procedure in conjunction with tissue culture (thermotherapy invitro regeneration).
It’s not all bad news because of its unique biology, Agrobacterium tumefaciens is often used in genetic modification of plants to improve food security.
Agrobacterium is a natural soil bacteria found everywhere. It is only virulent and causes crown gall when the Ti plasmid is incorporated in it. This is hard to test for in soil. Crop Nuts can test the Agrobacterium tumefaciens with Ti plasmid in plant material and in water (e.g. recycled water)
Till next time,
Ruth Vaughan is the Technical Director at Crop Nutrition Laboratory Services Ltd. (CROPNUTS). Ruth is also a contributing author to Kenya’s leading horticulture magazines such as the HortFresh Journal, HortiNews and Floriculture. Ruth is a great believer in soil health, organic matter, biochar and carbon sequestration as a way to alleviate climate change and increase food security. Loves visiting farmers and seeing all the different farming methods