The 2024 field season is in full swing, and samples have been coming into the clinic at a breakneck pace. As I think about the samples that have been submitted thus far, I’m struck by how may fruit samples I’ve looked at. Here’s a summary of the fruit diseases I’ve been seeing.
Apple scab
Wow! In my (almost) 26 years in the clinic, I’ve never seen a year as good for apple scab as 2024. We’ve had perfect weather for an apple scab epidemic. We had substantial rains around the time that trees were first leafing out, and this is the optimal time for initial infections to occur via spores released from apple and crabapple leaf litter. These early rains also prevented people from being able to make protective fungicide applications to their apples and crabapples (if they were so inclined). We continued to have rains throughout the spring, and that provided the leaf wetness needed to allow additional infections to occur via spores that were produced from the initial infections. I have received so many calls and received so many samples from clients complaining that their apple and crabapple leaves are turning blotchy and brown, and then falling from trees. This is virtually all due to apple scab. Expect to see more leaf loss as we head into the summer.
Cedar-apple rust
On apple and crabapple trees that haven’t defoliated due to apple scab (i.e., that are resistant to that disease), I have been seeing a lot of yellow/orange spotting. This includes one of the crabapples in my front yard. This spotting is caused by one of the Gymnosporangium rusts, most likely cedar-apple rust. Cedar-apple rust is a fungal disease, where the pathogen spends part of its life on junipers (where it produces a brown, brain-like galls that sprout bright orange, gelatinous growths) and part of its life cycle on apples and crabapples where it leads to yellow/orange leaf spots. I received several photos of the gelatinous juniper phase of this disease in mid-spring. We had a lot of rain around that time, so I’m not surprised to now see a lot of leaf spotting. Watch your apples and crabapples for this disease, and if you happen to see a variant where the spotting is red/fuchsia rather than yellow/orange, give me a shout and send me some photos. You could be dealing with a new Gymnosporangium rust (red star rust) that recently has been found in Wisconsin. I’m attempting to track this disease, so if you’re in a county where red star rust has not been formally confirmed, I’ll ask you to send in a leaf sample for testing.
Fire blight
I talked about this disease in my May web article (Wisconsin’s Wild Weather Woes), so check out that article for details. As I predicted, I’ve been seeing an uptick in the number of cases of fire blight in 2024. My most interesting fire blight cases have been on quince, a fruit crop that I rarely see.
Taphrina diseases
I have seen three of these diseases so far this season, caused by three different species of the fungus Taphrina. Two of the diseases have been on stone fruits. Peach leaf curl (caused by Taphrina deformans) leads to distorted (sort of bubbly-looking) and typically colorful (creamy white, light green, or pink) areas on peach and nectarine leaves. Plum pockets (caused by Taphrina pruni) affects plum fruits causing them to become enlarged and hollow. Early in the development of this disease, the fruits are spongy, but they eventually dry and become brittle. Finally, I’ve seen oak leaf blister (caused by Taphrina caerulescens), where the fungus leads to irregular, puckered areas on leaves. Management of severe cases of peach leaf curl and plum pockets (you can just ignore oak leaf blister) often involves making an application of a copper-containing fungicide to branches (where the fungus overwinters) after leaf drop in the fall or before leaf emergence in the spring.
Bacterial canker
You can think of this disease as being the “fire blight” of stone fruits (i.e., cherry, plum, peach, apricot). The bacteria involved in this disease can infect through flowers causing them to wither and die. This phase of the disease is often referred to as blossom blast. On branches and trunks, the pathogens often gain entry via wounds (e.g., cold injured tissues, pruning wounds), and you will typically see blobs of sap oozing from the infected areas. These blobs can range from large and obvious to small, subtle, and nondescript. Aggressive and timely pruning of infected branches is critical for management of this disease. Pruning will prevent movement of bacteria into the main trunk, which will ultimately lead to tree death.
Root/crown rots
I can see these diseases on virtually any fruit crop, but my most recent case was on raspberries. The client complained that his raspberries were stunted and had purplish foliage, two symptoms that I often associate with root/crown rots on raspberry. I tested root/crown tissue from his plants for Phytophthora and sure enough, the plant tested positive. Phytophthora (a type of fungus-like water mold) is the most common organism that I see causing root/crown rot issues on raspberries. It produces thick-walled resting spores that can survive for years in the soil and becomes particularly active and problematic when soils are wet. Unfortunately, there are few management options for home raspberry growers because of how aggressive this pathogen is and how long it can survive in a garden. My typical recommendation is to establish a new raspberry patch in a new location with new plants.
Hopefully, your fruit crops are healthy and growing well, but if not please contact me at (608) 262-2863 or pddc@wisc.edu for advice. For more information on plant diseases in general and their management, be sure to check out the UW-Madison PDDC website (https://pddc.wisc.edu/). To keep up to date on clinic activities and resources, follow the PDDC on Facebook or Twitter (@UWPDDC), or subscribe to the clinic’s listserv, UWPDDCLearn (by emailing or phoning the clinic to subscribe).