By Matt Gibson & Erin Marissa Russell
Fire blight disease leads to vascular wilt infections in pome trees, like apples and pears, and members of the Rosaceae plant family. Rosaceae is a large family that includes trees, shrubs, herbs and ornamentals, including mulberries, figs, strawberries, and roses. Vascular wilt disease sends its pathogens to clog up the works inside plant tissues so that water, as well as essential nutrients, cannot travel freely through the plant. As the plant is cut off from hydration and nutrition, it gets weak, begins to wither, and eventually dies as a result.
Fire blight is one of the biggest and most destructive plant diseases that threatens pear and apple trees. Aside from pome trees, fire blight also affects loquat, cotoneaster, and pyracantha plants, among other ornamental plants. Fire blight can kill branches, create water-soaked flowers, discolor leaves and bark, and even kill entire plants.
Fire blight is a bacterial disease. The bacteria that causes fire blight can be spread from plant to plant by insects, from splashing rain, and from using gardening tools, such as pruning shears, that carry the bacterial pathogens that cause fire blight. To manage the disease, plant resistant varieties, practice proper cultural control techniques, prune regularly, and as a last resort, treat plants with preventative chemical sprays.
How to Identify Fire Blight
Fire blight symptoms can be noticed on all above ground parts of the plant, including flowerheads, fruit, stems, branches, limbs, and the visible portion of the rootstock near the lower trunk.
On the blossoms, symptoms can first appear about one or two weeks after petal fall. The remaining flower parts start to look wet and dull, turning grayish-green or brown. Tissues then begin to shrivel and the color of remaining tissues begins to turn dark brown or black. At the base of the flower clusters and on young fruits, similar symptoms can be seen. In high humidity environments, droplets of bacterial ooze begin to form on fruit and discolored plant tissue. The ooze starts out milky-white and turns amber over time.
On infected shoots, symptoms appear similar to those found in flowers but spread and mature faster. Tips of shoots wilt quickly, forming a, “shepherd’s crook.” The leaves on affected shoots appear blackened along the midrib and on the veins. When several shoots on a tree are affected, the tree starts to look burnt or blighted, which is where the disease gets its name.
On the leaves of affected plants, infections appear as darkened or water-soaked spots. Sometimes spots can appear sunken or depressed. The bark of young branches also begins to darken and can look water-soaked. As the disease advances to later stages, bark can start to crack and surface areas become depressed or sunken in. Plant sap mixes with bacterial ooze and seeps out of the cracks. The wood beneath affected bark will appear streaked, with brown or black discoloration.
On the surface of fruits, water-soaked sores start to form and eventually turn dark brown or black. Bacterial ooze forms droplets that emerge from lesions. Heavily damaged fruits turn black and shrivel.
Signs of infection even appear on the rootstocks near graft unions. Bark near the rootstocks start to appear water-soaked and turn purple to black. Bark eventually cracks and bacterial ooze starts to emerge from cracked areas. Wood beneath affected bark near the rootstock develops reddish-brown to black streaks. The symptoms of fire blight are sometimes confused with phytophthora collar rot.
How to Prevent Fire Blight
While fire blight can cause losses of crops and be extremely negative for trees, it’s an avoidable illness. Below you’ll find a list of ways you can prevent the disease from taking hold in your garden. Choose the methods that will fit best with your property’s characteristics and with your schedule, then implement as many of them as you can to help keep fire blight at bay.
- While complete immunity is not possible in apples, moderately resistant cultivars can be found on the market. Gardeners of pears have fewer choices simply because there are fewer varieties of pear available. Make your life easier as a gardener and go for resistant varieties when you’re choosing new members of your collection.
- Using too much fertilizer or watering trees to excess will make your trees more susceptible to fire blight. Overages of nitrogen fertilizers especially contribute to increased risk of fire blight. Don’t make it easier for this disease to take hold—provide only the needed amount of water, and don’t go overboard with fertilizer. Remember to include rainfall in the total amount of water trees receive, using manual watering as a supplement.
- Anything you can do to avoid wounding trees with gardening tools or otherwise causing mechanical injury will help reduce the risk of secondary infection. Additional sources of secondary infection that you should work against in the garden include the presence of plant bugs and psylla, use of limb spreaders in fairly new orchards, and irrigating with overhead sprinklers.
- When you’re watering your trees, be careful not to splash the foliage or branches with water. Instead, aim for the base of the tree, watering the surface of the soil where the roots lie underneath. This is important because splashing water onto your trees can exacerbate the damp conditions that lead to fire blight. Also, water laying on top of foliage can’t be absorbed by the plant’s roots and put to use. However, it can be heated by the sun and cause sunscald.
- Keep the garden clean to avoid plant debris or leaf litter from providing a home for the fire blight pathogens. It’s especially important to clean the surface of the soil before winter sets in, as the plant debris gives the disease a place to hide out over the winter so it can emerge to strike your trees again the following season.
How to Treat Fire Blight
Even the most careful gardener can end up facing an outbreak of fire blight in their garden. If this happens to you, you don’t have to just throw in the towel and accept the disease’s presence. Fire blight can be controlled with several different treatment measures. Below, you’ll find instructions for many of these ways to treat fire blight. However, be advised that although these treatment measures will minimize the effects of the disease, there is no way to cure fire blight, so prevention is of the utmost importance.
- When your trees are dormant in the winter, take the opportunity while you’re pruning the trees to remove any twigs and branches that show signs of fire blight When you remove infected areas, make your cuts four to six inches before any spots with cankers or other apparent symptoms. You can also do summertime pruning to treat a fire blight outbreak, making your cuts eight to 12 inches from the edges of cankers. Avoid cutting off large branches or making other cuts that will leave large wounds on the tree, providing fire blight with a convenient entry point. Don’t include the diseased twigs or other debris from infected trees in your compost bin, or you’ll risk re-introducing the infection to your garden once the mature compost is used. Instead, discard what you’ll throw away with burning. Remember to always use clean, sterilized garden tools, wiping them down with rubbing alcohol in between plants.You can use a solution of one part bleach to nine parts water to clean and sterilize your garden tools.
- In gardens with severe cases of fire blight, you may consider slowing the spread of the disease and the number of cankers by taking any measures you can that will slow down the overall growth of the trees. Things you can do to slow tree growth include cultivation, withholding water, and using nitrogen fertilizers. These practices are only recommended in gardens where fire blight has become quite severe, as they are detrimental to the overall health of your trees.
Fire blight can also be addressed using fungicidal treatments or chemical control measures. However, these options can be detrimental to the population of beneficial insects. Copper blossom sprays may be used when flowers first begin, but they can harm fruit and are not that effective. For this reason and others, many gardeners choose not to use fungicides or chemical pesticides. At any rate, as long as you choose a few preventive methods and survey your trees on a regular basis so you’ll notice any potential outbreaks and be able to resolve them quickly.