George Sundin...In this article, I will review what we’ve observed and learned in 2009 and try to explain mechanistically the cause of disease issues we’ve faced this year. Before that, I want to acknowledge the excellent MSU Extension folks in the state (Mira Danilovic, Amy Irish-Brown, Erin Lizotte, Mark Longstroth, Nikki Rothwell, Phil Schwallier, Bill Shane, Bob Tritten) that work long and hard to stay on top of things in the tree fruit world. I am indebted to these dedicated professionals, rely on them for information, and enjoy the many “fruitful” discussions we have.

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Bill Shane...In July 2006, Michigan Department of Agriculture (MDA) inspectors detected plum pox virus, a stone fruit disease previously unknown within Michigan and detected in only one other state in the United States.

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Bill Shane and George Sundin... This article is intended to serve as a mid-season review of bacterial spot, an important disease of peach and nectarine caused by Xanthomonas campestris pv. pruni.

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Michigan Department of Agriculture (MDA) announced Tuesday, July 14, that exotic blueberry viruses have been detected at several locations in west Michigan. The two viruses, known as blueberry shock and blueberry scorch, were found through routine surveys conducted by both MDA and Michigan State University plant pathologists. Both viruses are known to occur in certain West Coast and mid-Atlantic states where they cause significant losses to blueberry producers. These finds are the first detections in Michigan.

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By Annemiek Schilder and Mark Longstroth...As a long-established blueberry growing region, Michigan has had it share of virus diseases, such as shoestring, necrotic ringspot, leaf mottle, etc. Recently, two new blueberry viruses were found in Michigan. The diseases they cause are not new since they are present in other growing regions such as the Pacific Northwest, but they are new to Michigan. Fortunately, the infections appear localized and efforts are underway to eradicate them to protect the Michigan blueberry industry.

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By Annemiek Schilder... In the Midwest, angular leaf spot (also called bacterial blight) is the only reported strawberry disease caused by a bacterium, namely Xanthomonas fragariae. This bacterium has been hitchhiking around the United States on strawberry planting material, since it was first reported in Minnesota in 1960.

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By George Sundin... 2009 has already gotten off to a horrendous start regarding apple scab, first with the realization that resistance to strobilurin fungicides is present in Michigan and second with the extensive amount of rain and number of scab infection periods. Scab lesions are showing up now where the fungus was not effectively controlled during infection periods that occurred in late April.

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By George Sundin... A Special Local Need [EPA 24(c)] registration has been granted for the use of increased rates of Indar 2F for brown rot control. The basis for this request was from our observations that populations of the American brown rot fungus (Monoliniafructicola) are shifting in Michigan towards decreased sensitivity to sterol inhibitor fungicides (see March 31 Fruit CAT Alert article). Note: this special registration only applies to Indar 2F.

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By George Sundin... EPA has granted a Section 18 Specific Exemption for the use of Kasumin 2L (kasugamycin) for the control of the blossom blight phase of fire blight. This use is for orchards where streptomycin-resistant fire blight bacteria are present.

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By Timothy Miles and Annemiek Schilder... Now is the time of year to begin scouting for mummy berry. Mummy berry is a serious disease of high bush blueberries in Michigan and is prevalent in most blueberry-growing regions throughout the United States.

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Annemiek Schilder...Extended periods of wet weather spell feast for fungal plant pathogens, since they are highly dependent on moisture for spore dispersal and plant infection. While dry spells earlier this spring might have threatened to create a “famine” year for fungi, the tables have indeed turned. Repeated or continuous wetting of infected tissues over several days is particularly conducive to spore production as it allows thorough wetting of infected canes or other overwintering plant parts and promotes spore release.

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Annemiek Schilder...This is the time of year that virus symptoms become apparent, particularly in blueberries. A cool spring and slow growth also brings out symptoms more than warm weather. Virus and virus-like diseases in plants manifest themselves in several ways and can be easily confused with abiotic disorders like nutrient deficiencies and herbicide injury.

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By Mark Whalon, Renee Pereault and David Epstein... Plum curculio over-winter as adults in the soil, litter and ground cover trash in orchards and in surrounding areas. As soon as daytime and evening temperatures exceed 60ºF, Plum curculio will begin to move especially when a light, misty rain or humid night co-occurs with early spring warming trends.

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Nikki Rothwell, Erin Lizotte and George Sundin... Bacterial canker is caused by the bacterium Pseudomonas syringae, and this pathogen can infect sweet and tart cherry and plums throughout Michigan.

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By Mike Haas and Larry Gut... If you didn’t get the results you expected from your pear psylla management program last year you aren’t alone. Insecticide resistance is common in psylla throughout the pear-growing areas of the world.

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By Mark Longstroth... Dormant sprays of lime sulfur, Bordeaux mixture (copper and lime) and heavy dormant oils used to be an important part of fruit culture.

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George W. Sundin, Gayle C. McGhee, Amy Irish-Brown and Bill Shane...Streptomycin resistance in the fire blight bacterium Erwinia amylovora was originally detected in the early 1990s in Michigan, mostly in Van Buren County and in a few isolated orchards in Kent and Newaygo counties. Most of the resistant strains contained an extra sequence of DNA that encoded streptomycin-resistance genes. This distinguished the mechanism of resistance from that found in E. amylovora from the Pacific Northwest, in which the strains became resistant to streptomycin due to a chromosomal mutation. In the mid-to late-1990s, the streptomycin resistant strains from Southwest Michigan slowly spread westward towards the lake, and resistance was noted in an isolated orchard in Kent County.

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Annemiek Schilder...While fire blight is most common in pears and apples, it also affects raspberries and blackberries (Rubus spp.). Summer red raspberries cultivars K81-6 and Boyne are particularly susceptible. Losses result from berry necrosis and from tip dieback of primocanes. Fruit losses of 65 percent or more have been reported on thornless blackberries in Illinois.

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Nikki Rothwell...A lot of Ethrel damage has been showing up in the past few weeks in the northwest (view images). This injury has come as a result of highly stressed trees, most from drought stress. However, we have seen San Jose scale on some sweet cherries in certain areas, and these insects have put more stress on trees. This stress is then compounded when we apply Ethrel, especially under hot temperatures. Injured trees have lots of leaf yellowing and leaf drop, and there is excess gummosis in affected trees. The gummosis is often clear, and it can be anywhere on the tree – branches, trunks and branch angles.

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Nikki Rothwell and George Sundin...Cherry leaf spot is the most important fungal disease of tart cherry in Michigan. The leaf spot fungus Blumeriella jaapii infects leaves with symptoms first appearing on upper leaf surfaces as small purple spots. As spots accumulate on leaves, the leaves turn yellow and fall. The amount of lesions required to cause leaf yellowing and drop is variable. Late summer (August, early September) defoliation reduces the ability of trees to store photosynthate in roots leading to an overall loss of vigor and leaving trees more susceptible to killing by winter injury. Early-defoliated trees also typically exhibit reduced flower bud formation and often set less fruit the following season.

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Mark Longstroth...The question of pruning out fire blight strikes during the growing season, or leaving them until the winter is debated whenever we have wide spread fire blight. The answer to this question depends upon the number of strikes and the weather conditions suitable for infection. Generally, pruning out fire blight strikes only makes sense in young orchards and when there are a relatively small number of strikes that are easy to get to.

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Larry Gut, David Epstein and John Wise...
Scout to determine need for control measures
The availability of several excellent obliquebanded leafroller control materials has growers asking advice for proper timing and sequencing of these materials, but before insecticide applications are undertaken, growers are best advised to first determine whether such applications are necessary. Due to the difficulty in scouting for overwintered obliquebanded leafroller larvae in early spring, obliquebanded leafroller insecticides are routinely recommended as part of an early spring spray program on farms with historical leafroller damage.

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Bill Shane and Mira Danilovich...In July 2006, plum pox virus (PPV) was detected in a single plum tree sampled at the MSU Southwest Michigan Research and Extension Center (SWMREC) in Benton Harbor, Michigan (view images). PPV is a serious disease of stone fruit that can cause significant yield and quality loss. PPV has been the most significant disease of stone fruit in Europe for many decades and was detected for the first time in Pennsylvania in 1999, Ontario in 2000 and in New York and Michigan in 2006. The Michigan Department of Agriculture (MDA) has worked very intensively since that time to test the Prunus trees at risk at this and other Michigan locations. After testing more than 50,000 samples in late summer, no new “positives” were found in Michigan.

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Christy Sprague...In the May 3, 2007 Field Crop CAT Alert, Steve Gower with MSU Diagnostic Services reported the first confirmation of glyphosate-resistant horseweed (or marestail) in Michigan. Steve has been actively screening weed samples for glyphosate-resistance over the last five years. We have been extremely lucky in Michigan, because of all of the samples that Steve had tested over the years none had been confirmed glyphosate-resistant until now.

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George Sundin and Nikki Rothwell...Cherry leaf spot is the most important fungal disease of tart cherry in Michigan. The leaf spot fungus Blumeriella jaapii infects leaves with symptoms first appearing on upper leaf surfaces as small purple spots. As spots accumulate on leaves, the leaves turn yellow and fall. The amount of lesions required to cause 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.

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George Sundin...Fire blight is caused by the bacterium Erwinia amylovora, an organism that is capable of infecting blossoms, fruits, vegetative shoots, woody tissues and rootstock crowns. There are several distinct phases of the disease including blossom blight, shoot blight and rootstock blight. The diversity of host tissues, combined with the limited number of management tools available to control the disease, has made it difficult to stop or slow the progress of fire blight epidemics.

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Mark Longstroth...Peach leaf curl has become an important disease in Michigan. This disease of peach and nectarine can defoliate trees. Infections take place in the spring as the buds open. The fungus infects peach buds from bud swell to bud opening under wet conditions. Air temperatures between 50 to 70°F are ideal. Rain or dew moves spores into the opening bud allowing the infection of young tissue.

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Bill Shane...Following the discovery of plum pox virus (PPV) on a plum tree at the SW Michigan Research and Extension Center in late July 2006, all susceptible trees at SWMREC were tested by ELISA. The original plum tree is still the only positive for SWMREC. The positive tree has been destroyed

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The Michigan Department of Agriculture and Michigan State University (MSU) today announced that aggressive efforts are underway to manage the plum pox virus (PPV), a plant disease known to infect certain stone fruits, such as peaches, nectarines, apricots and plums. The strain identified is not known to affect cherry trees. The U.S. Department of Agriculture confirmed plum pox virus in samples collected from a routine survey at the MSU Southwest Michigan Research and Extension Center (SWMREC), near Benton Harbor. PPV poses no human or animal health threat.

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Annemiek Schilder...This is a good time to check blackberry and black raspberry plantings for orange rust. Red raspberries are immune. Characteristic symptoms are spindly shoots with clustered, misshapen, pale green to yellowish leaves, as well as bright orange, powdery blisters on the undersides of leaves. Before the blisters burst open, they look waxy or shiny, as if covered with lacquer. On black raspberries, the rusted leaves start to wither and drop in late spring to early summer.

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George Sundin...It appears that most areas of the state did not experience weather during bloom conducive to blossom blight infection by the fire blight pathogen. However, temperatures will eventually warm and fire blight cankers will begin to ooze. Thus, inoculum will be present throughout the state.

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George Sundin and Nikki Rothwell...Cherry leaf spot (CLS) is the most important fungal disease of tart cherry in Michigan. The leaf spot fungus Blumeriella jaapii infects leaves with symptoms first appearing on upper leaf surfaces as small purple spots. As spots accumulate on leaves, the leaves turn yellow and fall. The amount of lesions required to cause leaf yellowing and drop is variable. Sweet cherries can tolerate quite a few lesions before leaf drop occurs, but Montmorency tart cherries will drop with only a few lesions, signifying the importance of proper leaf spot management. Balaton® trees can tolerate more lesions that Montmorency, but they, too, drop their leaves more readily than sweet cherries.

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Nikki Rothwell and George Sundin...With the continuing onslaught of rain, growers are wondering about the best options for cherry leaf spot control. Because the rain has provided so few windows for fungicide applications, there have been varying amounts of fungicide applied to tart and sweet cherry blocks. As rain is expected to continue for the remainder of the week, growers need to make quick decisions about the chemistry to apply when the rain actually stops for a while and the wind dies down enough to apply a spray.

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Mike Haas and Larry Gut...Pear psylla management can be difficult, especially if you do not have a program outlined for the season. If control measures are not taken early in the spring, the summer may be spent trying to get a handle on this pest. A little planning can go a long way toward limiting your commitment of time and money over the course of the season. Knowing a little about pear psylla biology provides a foundation for sampling and timing of control measures.

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