Current disease issues - E Letter
Even though we are officially still in moderate drought (US Drought Monitor, http://www.ncdrought.org/), many of us have experienced more than average rainfall over the past three months. In general that’s a good thing, and no one is complaining. Garden centers do notice that rainy week-ends affect sales, and rainy week-ends in spring can seriously affect end of year profit or loss. And rainfall can affect plants adversely by improving the growing conditions for diseases. There are a couple of diseases being reported recently that it would be good to look for before they get further advanced.
The first is affecting tomatoes, the other apples and pears. The frequently reported tomato problem is early blight. This is a fairly common and predictable disease that most gardeners learn to deal with. Early symptoms include small brown spots with a yellow halo, usually on the lower foliage. As the spots enlarge, concentric ring patterns may be observed and the entire leaf may turn yellow. Eventually large portions of the leaf turn brown. The symptoms gradually progress from the bottom up through the plant. Most infections of the early blight fungus (Alternaria solani) come from the soil. The pathogen survives in the soil and on plant debris. This is a good reason to use the oldest and still one of the most effective disease control measures we know – rotation. Don’t plant tomatoes in the same place that you have planted tomatoes (or potatoes, eggplant, peppers, petunias) in the last 3-4 years. Longer is better. When the pathogen is present in the soil, rain (or irrigation) hitting the soil can cause it to splash up onto the plant. A layer of mulch can help reduce infection. Optimal conditions for the pathogen to grow and thrive are when the temperature is in the range of 75-85 (sound familiar?) although it can grow more slowly from a range of about 40-100. Once the pathogen is established on the lower portions of the plant, water splashing through the foliage spreads it to other leaves or stems. As these leaves turn yellow to brown and die, they no longer provide adequate shading for the fruit which may result in sunscald. Most gardeners learn to remove the affected lower leaves to reduce the spread of the pathogen up through the canopy. Healthy, vigorous plants can usually tolerate some disease with little loss of productivity. Actions that gardeners should take are rotation every year, mulch around the plants, removal of affected leaves, and keeping plant vigorous and healthy.
The problem that people are reporting on apples and pears is fire blight. Fire blight is caused by a bacterium, Erwinia amylovora. It is the most serious disease of pears and apples killing blossoms, fruiting spurs, branches, and entire plants. Symptoms commonly observed are leaves and stems that turn brown or black and stay on the stem. There is usually a wilt of the terminal stem resulting in a “shepherd’s crook.” The bacterium survives in cankers on the stems – a good justification for pruning the 3-Ds during winter: anything dead, diseased, or damaged. You have to look for the small wounds on the stems. It may also overwinter in mummy fruit that are left at the end of the season. These should always be removed. New infections occur when rain splashes from the canker to flowers. You see the impact of rain during bloom period on the incidence of disease. It is also spread by insects who visit both the bacterial ooze on the canker as well as the flowers. Pollinators who visit diseased blossoms further spread the pathogen to other blossoms they visit on that tree or others. Splashing rain leads to secondary spread. Other wounds on the plant are also points where the disease may be introduced by water running down stems or by insects. Once the pathogen is introduced, it continues to move through the conductive tissues down through the stems to larger branches and sometimes into the trunk. Besides winter pruning to reduce overwintering sites for the pathogen, a primary control strategy is to remove any infected plant parts promptly in spring and early summer. There is no cure for this disease except to surgically remove affected plant parts.
Now is a good time to inspect the trees and prune out any infected shoots. Pruning cuts should be made approximately 12 inches below any visible signs of infection. Pruners should be sterilized between cuts to reduce the incidence of spreading the bacterium on tools. Sterilize tools with isopropyl alcohol or household bleach diluted with water at a rate of 1 part bleach to 9 parts water.
I hear two objections to pruning out the diseased portions:
1) it’s too high to reach and
2) there won’t be anything left.
1) If it is fire blight and it’s too high to reach and you wait, it will get low enough for you to reach. You can prune lower now or wait till it gets worse. If it’s fire blight, it will get worse.
2) If it’s fire blight and you don’t remove it, eventually there won’t be anything left anyway. The bottom line here is that if you have fire blight in your apple or pear (or even pyracantha) it needs to be removed. It doesn’t get better. After pruning, pay attention to see if you’ve gotten it all. If you have more evidence of infection, keep pruning.
alcooke