What Is Apple Blotch Fungus: Tips For Treating Apple Tree Fungus
Apples from your own tree are one of the greatest rewards your garden can provide. But what do you do if your apples look a little less magnificent than the ones at the market? There are several treatments for apple blotch fungus disease, so read on to learn more.
What is Apple Blotch Fungus?
Apples are a beautiful addition to the home orchard and also work wonderfully as standalone plants in the landscape. Growing apples, however, isn’t as easy as growing other hardy perennials. If you want your apples to thrive and produce lots of fruit, you’ll want to pay close attention to their care throughout the year. Apple blotch fungus disease is just one common problem for both apple farmers and homeowners alike.
Blotch fungus on apples is a common disease caused by a variety of fungi throughout the fruiting season. Fortunately, it’s a problem that’s limited to the skin of the apple. It’s also safe to eat unless you have a mold allergy, so for many homeowners, apple blotch fungus disease may not pose a serious enough threat to treat. For others, some level of treatment between none and orchard-level protection may seem more appropriate.
Apple blotch symptoms usually present as quarter inch (0.5 cm) or larger irregular areas on the surface of infected fruits. The color may be cloudy or sooty, often making the apple surface appear olive green. It’s common for smaller areas to come together to form larger, non-circular spots on the skin. Apple blotch fungus disease is sometimes accompanied by a similar fungal disease known as “flyspeck,” which will add small, raised black spots in addition to the sooty blotches.
Treating Apple Blotch Fungus
If the blotch is minimal and the fruit’s appearance is acceptable, fruits can usually be eaten after a vigorous rub of the skin. Complete skin removal for baking or juicing will also eliminate the need to take special effort against the fungus on your trees. Gardeners wanting to do more can remove nearby bramble patches to help destroy common vectors for apple blotch fungus.
Pruning your trees aggressively in the winter can also be a huge help, since opening up the canopy means reducing internal humidity for your apple fruits. A good yearly prune also gives you better access to the fruits if you were to choose to spray them later.
Growers looking for more hands-on control methods may want to start by watching their fruits carefully in the spring. Infection can appear any time after the apple flower petals fall and the fertilized fruits begin to enlarge. If you notice blotches on the fruit, thin them while they’re small to prevent fungal transmission. Properly thinning your apples will both encourage larger fruit development and discourage a variety of pathogens, including apple blotch.
When treating apple tree fungus becomes a necessity, you have a few choices. You can apply a cover spray of fungicide as soon as the apple flowers begin to fall, then begin counting the hours your tree leaves are wet from rain or dew. At 175 hours, you’ll want to apply a second cover spray and then start applying a protective fungicide every 10 to 14 days throughout the growing season.
Fungicides containing thiophanate-methyl mixed with a contact fungicide, like captan, have been shown to be highly effective in orchard settings, but for homeowners, alternating sprays of kresoxim methyl or trifloxystrobin with thiophanate-methyl will provide good protection. Natural fungicides like sulfur spray are not effective against apple blotch fungus.
Trees can get infected with apple scab any time during the growing season. Depending on when a tree is infected, the symptoms are different.
The first signs of apple scab occur on the underside of leaves: dull, olive-green round spots, often along the leaf veins and on the leaves closest to the buds. These spots gradually grow into brown or black large circles with a velvet-like surface up to half an inch across.
As the disease progresses, the leaves may also become dwarfed or twisted, then turn yellow before falling prematurely in mid-summer.
The first signs of the fungus on the fruit are olive-green spots that gradually turn into brown or black lesions with a rough, wart-like or corky surface, and cracks in the fruit. The immature fruit might also drop.
Infections late in the season will lead to black spots on the apples. If the infection occurs just before harvest, these symptoms will only show when the apples are stored, hence the term “storage scab”.
Marssonina Leaf Blotch Causes Apple Leaf Defoliation Where Cover Sprays Were Stretched in Summer 2018
Marssonina leaf blotch is a disease of apple leaves and fruit caused by a fungus Marssonina coronaria (teleomorph Diplocarpon mali). In NY we are still working to confirm this species by reliable diagnostic methods. It is a problem in both conventional and organic apple orchards. Leaf symptoms express as standalone blotch spots that can coalesce, or more dispersed smaller spots. Leaf symptoms are more prevalent then fruit symptoms (we haven’t found fruit symptoms in NY). When leaf infections are severe due to lack of fungicide cover sprays, they lead to leaf yellowing and lower crown defoliation (Fig-s 1-5). This fungus caused defoliation of apple trees last year on Mutsu, Honeycrisp, NY-1, NY-2, Rome, Northern Spy, Gala, Winesap and other cultivars, after summer cover sprays with fungicides were not tightened, i.e. applied at shorter intervals, in alignment with frequent rains we got in 2017. In 2017, first symptoms were visible at the end of August and beginning of September. In 2018, the disease started expressing around 5-15 September in blocks where again summer cover sprays with fungicides were not tightened, i.e. applied at shorter intervals, in alignment with frequent rains we got in July, August and September 2018. We found typical symptoms on ‘Rome’ and ‘Mutsu’ leaves today. The fungus requires prolonged leaf wetness i.e. high relative air humidity for successful infection. Typical symptoms express 40
45 days after infection (Lee et al. 2011). There are no labelled fungicides for control of Marssonina leaf blotch in NY. Research indicates that thiophanate-methyl (Topsin M), DMI fungicides, combination of a QoI and SDHI fungicide, are effective but have no labelled use for Marssonina Leaf Blotch in apple so far in the USA. EBDC fungicides are effective but have no labelled use for Marssonina Leaf Blotch in apple and are limited with 77 days pre-harvest interval (PHI). Recent research from Germany (Bohr et al. 2018) shows that for organic orchards 10-12 sprays per year of each product: acid-clay Myco-Sin, or Funguran (copper hydroxide), or Curatio (lime sulfur), or sulfur, provide good control if first spray application is started around 10-12 June.
Figure 1. Typical Marssonina leaf blotch spot and leaf yellowing on ‘Mutsu’ leaf (Photo by Acimovic S. G. 2018). Figure 2. Typical Marssonina leaf blotch spots and leaf yellowing on ‘Mutsu’ leaf (Photo by Acimovic S. G. 2018). Figure 3. Typical Marssonina leaf blotch sheet of spots and leaf yellowing on ‘Mutsu’ leaf (Photo by Acimovic S. G. 2018). Figure 4. Typical Marssonina leaf blotch symptom spot and leaf yellowing on ‘Mutsu’ leaf (Photo by Acimovic S. G. 2018). Figure 5. Marssonina leaf blotch defoliation of the lower part of the crown on severely infected ‘Rome’ trees (Photo by Acimovic S. G. 2018).
When we examined the leaf spots we detected typical Marssonina sp. two-cell spores (Fig. 6).
Figure 5.Typical two-cell conidia spores of Marsonina sp. visible under the microscope and detected after examining the spots in Fig-s 1-4 (Photo by Acimovic S. G. 2018).
Bohr et al. 2018: Symptom occurrence and disease management of Marssonina blotch. 18th International Conference on Organic Fruit-Growing: Proceedings of the Conference, 19-21 February 2018, Hohenheim, Germany.