Plant Diseases

July 18

There is some fungus among us.  The rainfall and sometimes excess moisture has brought on a plethora of diseases not often noticed in the landscape.  Common diseases on trees and ornamentals are apple scab on crabapple trees, Diplodia and Rhizosphaera needle cast on certain conifer trees, and powdery mildew on lilacs and many other species.  This year there has been somewhat severe outbreaks of anthracnose on maples, white oaks and especially sycamore trees.  Scientists have developed varieties of crab apples that are mostly immune to apple scab.  A few more trees die every year from Dutch elm disease.  If you live in an area where oak wilt is present, have your trees treated professionally with Alamo fungicide to protect those trees.  One treatment lasts up to three years.  Make sure you treat desired trees near infected trees.  The disease passes from tree to tree through underground root grafts.

Now that the vegetable garden is growing you will see either septoria leaf spot or early blight on tomatoes.  Septoria starts with small dark bordered white spots while early blight has circular brown spots with margins of yellow; both start on lower leaves where those leaves die and the disease progresses up the plant.  Early blight will also be a problem on potatoes.  If weather remains cool and damp for a period of time potatoes and tomatoes will be susceptible to late blight, the disease famous for the Irish potato famine where 1.5 million people died in the mid 1800’s.  Powdery mildew will affect onions causing the leaves to die early.  A cercospera leaf spot will affect carrots and beets.  Of all garden diseases, early and late blight on tomatoes is probably the most damaging.  Downy mildew and anthracnose occurs on vines crops and a few other crops.  Several other diseases are caused by virus, vectored by insects, and bacterial spot diseases caused by soil splashing up onto leaves.  My raspberries grow in a very wind protected area with barely enough sun.  The disease, Botrytis fruit rot is common on most soft fruits.

Several species of vegetables, especially tomatoes get blossom end rot.  This disease is a physiological disease caused by a lack of calcium in plants.  As the plants go through a very quick growth stage and start to fruit the roots cannot mine from the soil and transport enough calcium to keep up with the growing fruits needs.  When the plant gets more mature it generally overcomes this disease on its own.  This disease can be alleviated with the addition of a foliar calcium spray from a product like Green Light’s Blossom End Rot Spray.  It and other similar products contain soluble calcium. 

The most common ornamental disease that is treated is rose black spot on roses.  Roses also get powdery mildew.   

The three best things you can do to keep diseases away is to choose plant varieties that are resistant to common diseases, plant in areas with good air movement to keep plants dry, and water in the morning so plants will be wet for the shortest period of time.  In order for a disease to grow it needs a susceptible host, the proper environment to grow in, and the causal agent.  This is called the disease triangle.  The next best item you can do is spray a fungicide.  The most common all around fungicides and the fungicides that controls the most diseases contain chlorothalonil.  It was originally named Bravo but now comes in all types of professional and consumer formulations.  Chorothalonil is a protectant and will not stop existing diseases.  It must be applied every 7 days.  Other common organic fungicides are Neem oil and the elemental copper and sulfur fungicides.  The elemental fungicides work well but must be reapplied often and at heavy rates every 5 to 7 days.  There are some diseases they do not control.  Neem oil is a fair protectorant fungicide and can stop powdery mildew.  It should be applied every 14 days to protect vegetation and every 7 days when a disease exists until it is controlled.  Neem oil also leaves a pretty shine on the leaves and fruit.  There are some new fungicides on the market that can be used on some fruits and vegetables.  They not only protect but have the ability to burn out infections up to four days after the infection starts.  These fungicides have not made there way to many consumer products and are expensive. 

When treating edible fruits and vegetables, pay attention to the preharvest interval (PHI).  Neem oil can be used up to the day of harvest.  Others have various intervals between treating and harvesting of produce. 

My most favorite disease is a lawn disease called fairy ring. This scientist researched how to control it for several years.  It grows as an ever expanding circle of green or sometimes dead grass and under certain conditions produces a ring of mushrooms.  Fairy ring has several different causal fungi and is one of the least understood of all diseases.  Every year the disease just grows a little more and a little more until some circles become very large.  One scientist found one that he followed for several miles.

Best of luck with disease control.

The Garden Doc