Overview of small fruit diseases during the 2008 growing season

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

The 2008 season was challenging for small fruit growers, as frequent precipitation and fluctuating temperatures promoted many fungal diseases, especially those that rely on rain for spore dispersal and infection. At the same time, the inclement weather did not allow growers to apply fungicide sprays at the optimal time and also led to washing off of fungicides that were applied. Together, these factors made for a less than optimal season for fruit growers.

Blueberries

Snow cover over the winter provided an ideal habitat for overwintering mummy berries, with sufficient moisture for mummies to germinate in the spring. After a cold period in April which delayed germination and apothecium development, conditions turned more favorable. The weather was conducive to the development of shoot strikes, which were first noticed in early May. Conditions during bloom were good for dissemination of spores to the flowers by bees due to an extended bloom period. Fruit infections were also severe, particularly in sites with a history of mummy berry and in unsprayed or insufficiently sprayed areas. In a fungicide efficacy trial, we saw extremes of 115 shoot strikes and 579 mummified berries per bush in unsprayed plots – a record. Some growers may have been taken by surprise by the level of mummy berry infection as levels have been relatively low over the past five years. Mummified berries were even noticed in clamshells with Michigan blueberries sold in supermarkets this summer.

Anthracnose fruit rot incidence was moderate this year , it appears that the cold spring and early summer limited sporulation and infection. Alternaria, Botrytis, and Phomopsis were also found affecting fruit in post-harvest rot tests.

The cold wet weather and freeze events also led to bacterial twig blight infections (bacterial canker) caused by Pseudomonas syringae. It was characterized by dark brown to black twigs, which at first sight looked like Phomopsis twig blight. In some fields, the dark-blighted twigs were more common in lower lying areas and occurred despite a tight fungicide program. However, no fungi were isolated from these twigs and bacteria streamed out of the vascular bundles after incubating the twigs in moist chambers. This was the first time that we have seen this disease in Michigan. Cold, wet conditions and frost injury promote infection. While Phomopsis cankers were apparent on last year’s canes (due to heavy rains in August of 2007), we did not see a lot of cane death or twig blight this season.

Virus and virus-like symptoms were more obvious in some bushes this year, which is typical in cool years. However, some odd symptoms were also noticed, like unthrifty bushes with purple blossoms in the spring and leaf reddening and necrosis later in the season. While it initially was thought that herbicide damage could have played a role in the development of leaf curing and necrosis, the patterns of affected bushes indicated a possible virus problem. The symptoms were also widespread in a many fields. Various ELISA tests were done on plant samples but were negative except for blueberry shoestring virus. Investigations are ongoing as to the cause of this baffling symptom.

Another rare blueberry disease in Michigan was seen in 2008, namely red leaf, which is caused by the fungus Exobasidium vaccinii and reduces growth and productivity. This fungus systemically invades plants and causes the leaves to turn fully or partially red. It is a very striking disease. Infected plants continue to produce infected canes every year.

Grapes

Due to the cool, wet spring and rainy summer, black rot and downy mildew were particularly prevalent on leaves and clusters in unsprayed or insufficiently sprayed vineyards this year. Black rot, Phomopsis, and downy mildew all need rain/wetness for spore dispersal and infection, so this season was particularly conducive to disease development as a whole. In most commercial vineyards, however, growers managed to control black rot and downy mildew well, despite the frequently difficult conditions for spray applications.

Downy mildew on fruit clusters and leaves of grapes showed up early. Regular rain events in late spring and summer encouraged infection. In the ‘Chancellor’ research vineyard at TNRC in Fennville, 100 percent cluster infection occurred in the unsprayed control. Downy mildew also got an earlier start in many ‘Niagara’ vineyards than in recent years. Most growers did apply fungicides for downy mildew. Some growers that had missed the opportunity to apply Ridomil because of the long PHI used ProPhyt or Phostrol for control.

Despite the rain, Phomopsis disease pressure was not as high as expected, possibly due to the cold weather in late spring and early summer, which can suppress sporulation and infection. However, cane, leaf, rachis, and fruit infections are still plenty common in susceptible cultivars, like Vignoles and Niagara. In vineyards where black rot was common, it may have outcompeted Phomopsis.

Powdery mildew showed up relatively late in most vineyards, but became severe in some wine grapes due to warm dry conditions prevailing in late summer. Cases of berry infection were reported in ‘Concord’ vineyards and most likely occurred due to ascosporic infection from overwintering inoculum after rain events right after bloom. This is the time when the berries are most susceptible. There was potential for severe powdery mildew due to early rains which would have promoted ascospore release. However, too much rain and cool weather may have slowed down powdery mildew in the early part of the summer. Once it has gained a foothold, powdery mildew prefers warm, dry weather, and frequent rains that may actually lower disease incidence by washing powdery mildew spores off the leaves and causing bursting of spores in water droplets. Leaf, rachis, and berry infections were noted in wine grapes in unsprayed areas. On the other hand, growers were more acutely aware of the problem and sprayed more diligently as well. Powdery mildew on ‘Concord’ leaves was less severe than in prior years and late enough to be of little consequence.

This has been a moderately favorable year for Botrytis bunch rot and sour rot so far. Dry weather in August has generally helped keep these diseases at bay. However, recent heavy rains may turn that situation around. Frequent rains promote bunch rots. Any wounds created by insects or cracking of berries in tight bunches can encourage Botrytis development. Tight-clustered cultivars also provide a moist environment for infection and sporulation, which further spreads the disease. Botrytis bunch rot can be distinguished from sour bunch rot by the presence of grayish brown spore masses at the stem end or along wounds in the berries, and the absence of the vinegary odor associated with sour bunch rot. In addition, sour rot often has fruit flies colonizing rotting clusters.

A relatively rare disease of grapes in Michigan, anthracnose, caused by the fungus Elsinoe ampelina, was again observed at several sites this year. The fungus primarily attacks table grapes, but can also infect ‘Niagara’, ‘Concord’, and ‘Vidal’ and ‘Frontenac’ wine grapes. Symptoms on the canes somewhat resemble those of Phomopsis, but lesions are typically more sunken with raised edges. On leaves, the center of older lesions drops out, giving the leaves a puckered and “shot hole” appearance. Lesions on green berries are reddish brown or grayish with darker margins, and do not expand much upon ripening. This disease is favored by cool, rainy springs.

Strawberries and brambles

Cool wet conditions favored foliar diseases of strawberries like common leaf spot, Phomopsis, and angular leaf spot, as well as leather rot on fruit of strawberries. Angular leaf spot caused blackening of calyxes in some strawberry fields. Leaf spot, spur blight, and anthracnose cane spot were commonly seen on raspberries. Fire blight also occurred on raspberry in some locations, killing back shoot tips and sometimes fruit clusters on raspberries. Botrytis gray mold on the fruit was also common, both on strawberries and raspberries. It was mainly a postharvest problem on raspberries.

Oddly shaped strawberries (button berries) were probably caused by tarnished plant bug if they occurred later in the season (on everbearing strawberries), although freeze injury of flowers may have to blame in June-bearing strawberries. Some fields suffered from frost injury where frost protection was not feasible.

White drupelets, usually indicative of sun scald were also noticed, particularly in tunnel-grown raspberries. Due to the warm weather, mites were a problem in tunnels. Fungal diseases are not very common in tunnels since there is rarely free water on plant surfaces to allow for infection. However, it is possible to get some Botrytis infection of fruit if the relative humidity is very high (greater than 95 percent) for multiple days in a row- Botrytis conidia can germinate under those conditions, although they prefer a film of water. Leaf distortion, crinkling and plant stunting resembling virus symptoms were caused by potato leaf hopper.

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