Glomerella leaf spot causes irregular spots on the leaves and can defoliate an orchard by harvest. Glomerella is showing up in Gala and Golden Delicious orchards in southeastern U.S., but it has the potential to move north if warm summers continue.
As the climate warms and new apple varieties are introduced, apple growers might have to deal with apple diseases they’ve not experienced before, warns Dr. Turner Sutton, plant pathologist at North Carolina State University.
Growers in the southeastern United States have been seeing more diseases in their orchards during the past 10 to 15 years, Sutton reported at the Great Lakes Expo in Grand Rapids, Michigan. Recently, they have had to deal with alternaria blotch and glomerella leaf spot, and are wondering if the new fungicides they are using are less effective than the older fungicides captan and mancozeb, he said.
Glomerella leaf spot is a new and "scary" disease of Gala and Golden Delicious, Sutton said. It looks similar to the physiological disorder necrotic leaf blotch. It causes irregular spots on the leaves, and every leaf in the tree can be affected. If uncontrolled, it can defoliate trees by harvest. It is caused by specific strains of Glomerella cingulata, which is the sexual stage of –Colletotrichum gloeosporioides, the cause of bitter rot on fruit.
Glomerella leaf spot was first reported on Gala in Brazil in the 1980s. It was then found in Tennessee, Georgia, South Carolina, and now North Carolina. Sutton said it has the potential to move further north if warm summers continue. The leaf spot strain causes a fruit rot identical to the bitter rot produced by strains that do not cause a leaf spot.
Alternaria leaf spot became a problem on Red Delicious in the Southeast in the late 1980s and early 1990s, Sutton said. The symptoms—small necrotic spots on the leaves—look similar to those of frogeye leaf spot or a hypersensitive reaction to cedar apple rust. In the early 1990s, the region lost 10 percent of its Red Delicious acreage because growers were unable to control alternaria, but it is a problem that growers have learned to live with and deal with, he said.
Identifying the cause of leaf spots can be difficult, he said. Each year, growers mail him leaf samples, hoping he can tell them the cause. However, an accurate diagnosis depends on clues from the orchard. For example, alternaria leaf spot tends to be relatively evenly distributed through the tree, affecting both young and older leaves, because the spores that cause the infection are airborne.
A hypersensitive reaction of cedar apple rust occurs in certain varieties of apples, most commonly Empire. In this case, small yellow dots (pycnia) can be seen in the centers of some necrotic spots, and affected leaves are usually found on groups of three or four leaves that were the most susceptible during infection periods.
Leaf spotting injury from the fungicide captan also tends to be seen on groups of terminal leaves—usually the youngest leaves on the shoot at the time when the product was applied. Captan damage is most common when the product is applied with or close to oil under slow drying conditions, particularly on Red Delicious.
Frogeye leaf spot, caused by Botryosphaeria obtusa, is associated with dead wood in the tree. Large numbers of leaves with spots on them can be found close to cankers or mummied fruit. "You have to look at the pattern," Sutton said. "If you send me these leaves in a Ziploc bag, I probably can’t tell. If you look for them in the orchard, very often you can figure out the cause of the particular problems."
Brooks fruit spot, a disease that occurs in Michigan on Rome, Stayman, and IdaRred apples, also can be confused with other diseases, Sutton said. It is caused by Mycosphaerella pomi. In the Southeast, it became more prevalent after the sterol-inhibitor fungicides were introduced, as they had no effect on the disease. "We had orchards with 50 to 60 percent of the fruit infected with Brooks spot," he said.
Sixty years ago, blotch (Phyllosticta solitaria), apple scab (Venturia inaequalis), and black rot (Botryosphaeria obtusa) were considered the most important diseases in the southeastern United States, but blotch has virtually disappeared as most of the varieties grown today are relatively resistant, and most of the fungicides available today are more effective than sulfur, lime sulfur, and copper, which were used in the 1940s.
However, Sutton worries about whether the new apple varieties that are being introduced will lead to a resurgence of blotch.
"Sooner or later, I think we’re going to plant something that’s susceptible to blotch, and it’s going to become a problem we haven’t seen in many years," he said. "It’s around. It’s lurking. It’s just waiting for a susceptible variety."
Black pox (Helminthosporium papulosum) is common in the Mid-Atlantic area, particularly on Golden Delicious. It causes small spots on the leaves and small circular lesions on the fruit that vary from the size of a pinhead to much larger. The disease is not controlled well by the QoI (Quinone outside inhibitor) fungicides Sovran (kresoxim-methyl), Flint (trifloxystrobin), and Pristine (pyraclostrobin, boscalid), Sutton said.
Necrotic leaf blotch is a problem throughout the world where Golden Delicious is grown, he said. It’s a physiological problem that is aggravated by gibberellin production during cloudy overcast weather in the summer and is managed with the use of heavy metal fungicides or sprays of foliar zinc nutrients.
Black rot is easy to tell apart from other fruit rots, Sutton said. Symptoms are black, circular spots that can be as small as a pinhead early in the season but enlarge as the fruit begins to ripen. It develops into a firm, brown rot, whereas bot rot (also known as white rot) is characterized by a very soft, mushy rot.
Infections by the bot rot fungus (Botryosphaeria dothidea) can occur throughout the season and can be a significant problem on Golden Delicious. Early symptoms include small tan lesions on the surface of the fruit, which rapidly enlarge. The rot extends into the core, and expands to affect the entire fruit. Affected fruit are often very soft and light tan in appearance, which is why the disease is called white rot. "It’s just like applesauce in a bag," Sutton explained. However, when bot rot develops later in the season, it is a firm rot and takes on a very different character. At that time, it can easily be confused with black rot.
Bitter rot is the disease that southern apple growers are most afraid of, Sutton said. It has become more of a problem since restrictions have been placed on the use of EBDC fungicides, such as mancozeb, during the summer. Fruit lesions are brown and sunken, often covered with masses of orange spores. Thousands of spores can develop on each fruit. Infections that occur later in the season can be hard to tell at first from bot rot. However, bitter rot lesions eventually form a distinct V-shaped pattern in the fruit flesh, whereas bot rot progresses towards the core and then radiates out, affecting the entire fruit.
As the climate warms over the next decade, many of the diseases considered to be southern diseases could become problematic in more northerly apple-growing regions such as Michigan, he said. Disease management programs used in the Southeast might need to be more closely followed in the northern United States.
"It’s very important when we think of managing diseases that we keep two things in mind," Sutton said. "We want to minimize the possibility that infections can occur by creating conditions less conducive to the disease and to protect the fruit and foliage from those pathogens. Cultural practices are the foundation of a good disease-management program. There are many things we can do to reduce the likelihood of disease occurring."
Leaf shredding is a good integrated pest management tactic that helps reduce the disease inoculum that causes alternaria blotch, Brooks fruit spot, and glomerella leaf spot, as well as apple scab, he said. Growers need to prune out dead wood and cankers during the growing season to minimize late-season infections. They also need to prune to keep the canopy open and maintain a good tree structure that will allow good spray deposition and rapid drying conditions.
During the summer, in the Southeast, fungicides must be applied every 10 to 14 days from petal fall to harvest. Because the disease complex is varied, tank mixes of two or more fungicides are commonly used.