Tart cherry blocks and, to a lesser extent, sweet cherry blocks have been hit hard by cherry leaf spot this season in Ontario. The frequent rains through the season created optimal conditions for the leaf spot fungus to infect while heavy rains also quickly scrubbed off any protectant fungicides that were applied and made it difficult for growers to get into orchards to reapply.
This disease is caused by a fungus that overwinters on fallen leaves on the orchard floor and produces spores in the spring. The primary infection period may last two to six weeks depending on conditions. Ascospores develop between 6 and 16°C with ascospore discharge increasing with temperatures between 8 to 30°C. A wet period of only a few hours is enough for spore germination. Development of visible lesions occurs in 5 to 15 days. Temperatures of 16-20ºC are most favourable for disease development.
Symptoms of leaf spot
Small purple lesions on the upper leaf surface can appear within 5 days if there are damp conditions and the temperature remains steady between 16 and 20°C. This incubation period could take as long as 15 days though if lower temperatures and drier conditions occur.
Following infection, masses of light pink to white spores are produced on the underside of the infected leaf. Conidia (spores) are dispersed from leaf to leaf by wind or rain and the infection cycle can be repeated several times during a single season, depending on conditions.
As spots accumulate on leaves, the leaves turn yellow and fall. The number of lesions required causing leaf yellowing and drop is variable. Sweet cherries can tolerate quite a few lesions before leaf drop occurs; however, Montmorency tart cherries will drop with only a few lesions, signifying the importance of proper leaf spot management. Less than 50 percent defoliation by early September is considered acceptable control in Michigan.
The most conspicuous symptom, especially on sour cherries, is the golden yellowing of older infected leaves before they drop off. Although this symptom does not occur every season, the spotting of infected leaves is always visible.
Spots like those on the leaves may also form on leaf petioles and fruit pedicels, causing fruit to ripen unevenly. Spots usually do not form on fruit.
When significant defoliation occurs before harvest, fruit can become soft and immature, have low soluble solids and ripen unevenly. Significant defoliation can be quantified based on the standard that at least two leaves are needed to effectively ripen each cherry on tart trees. Following two or more years with significant defoliation early in the season, trees are susceptible to winter injury due to the loss of photosynthates and therefore store carbohydrates in roots. Blossom production may also be reduced for at least two subsequent years.
Management – Earlier than we thought
For effective cherry leaf spot management, the first fungicide application is routinely made around petal fall to adequately protect the first fully-expanded leaves from fungal infection. The cherry leaf spot fungus infects leaves through stomata, and these parts of the leaf structure are not open and functional until the first leaves are mature. However, when Michigan sustained early and epidemic levels of cherry leaf spot, researchers determined that this strategy did not take the first bract leaves into account. Bract leaves open prior to bloom, and open bract leaves can be infected early, prior to petal fall.
There are two main reasons that early infection can result in a fungal epidemic:
Once fungal infection occurs in the tree, the fungus will produce tremendous spore numbers from lesions established on the leaves, and in most cases, there are usually more spores developing from lesions than ascospores that are coming up from leaves overwintering on the ground.
Spores from lesions on the leaves are much more likely to find new leaf targets within the tree than spores coming up from the ground. The distance from one leaf to another leaf on a tree is minimal, and the potential for spores to infect by moving from one leaf to an adjacent leaf is “easier” than for spores shot from ground level to hit the leaf target up in the tree canopy. This early infection from leaf to leaf can result in a major infection event.
From pink through shuck split, the fungicide of choice for cherry leaf spot management is chlorothalonil (Bravo ZN, Echo 90DF) because this is a broad-spectrum fungicide with excellent efficacy against cherry leaf spot.
After shuck split is a critical disease timing in tart cherry orchards. This is not only because warming temperatures favor cherry leaf spot spore discharge and infection, but also because other diseases such as powdery mildew become active. At first cover, fungicide applications must target both cherry leaf spot and powdery mildew.
Several fungicides are labeled for cherry leaf spot including captan, Bumper, Fitness, Jade, Tilt, Nova, Quash, Fontelis, Flint, Pristine, Luna Sensation and Regalia Maxx. Some of these are also labeled for post harvest treatment. With the advanced state of the leaf spot epidemic, if you choose to use a locally systemic fungicide, tank mix with a captan product a to reduce the risk of resistance developing. Resistance to the Group 7 Cantus (also in Pristine) has been documented in Michigan.
It is critical to protect the remaining healthy leaves on tart cherry trees from both leaf spot and powdery mildew. Choose fungicides that will provide protection from both diseases. Protection must continue through the post-harvest period. Minimizing environmental stresses on the trees will help to keep them as healthy as possible. Research in apple scab has shown that flail mowing fallen leaves and applications of urea to fallen leaves after leaf drop help to reduce the number of spores produced the following year.
Much of the information for this article was collected from these web pages: