Overview of small fruit diseases during the 2010 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 2010 season was challenging for fruit growers, including an early, warm spring punctuated by spring freezes, which damaged many juice grape vineyards. In some locations, hail also damaged crops. Precipitation varied significantly by location so it was difficult to make blanket recommendations for the growing region. In some locations, blueberry fields were too wet to enter and apply fungicides in a timely manner. Frequent rains also resulted less then optimal fungicide timing and wash-off of fungicides. Overall it was a very warm and humid growing season, with droughty conditions in July and August. The harvest of all small fruit crops started significantly earlier than previous years. Overall, these presented challenging conditions for plant growth and disease control.

Blueberries

Generally, there were higher numbers of overwintered mummy berries than in 2009. The germination rate was higher than in previous years (up to 40%) in wet sites. However, in dry locations, germination rates were low. In early April, 2010, mummies were found with small apothecia, about two weeks ahead compared to 2009, due to the relatively warm soil temperatures and sufficient moisture. Overall, infection risk in early April was deemed fairly low because most apothecia were less than 1-2 mm in diameter and 2 mm is the minimum size for release of ascospores. However, in the second week of April 2010, significantly more mummies with apothecia were detected and apothecia were larger, increasing infection risk. Blueberry shoots were also at a susceptible stage in their development. In the last two weeks of April, dry weather led to drying out of mummy berry apothecia. The first shoot strikes were seen during the last week of April and increased until the last week in May. Conditions during bloom were conducive to infection, however, rapid plant development and low bee numbers in some locations due to early bloom seem to have limited fruit infections. The first mummified berries were seen at the end of June and increased into July but started dropping off the bushes early. Overall, the number of shoot strikes was similar or somewhat higher than last year, whereas the number of mummified berries was lower than last year.

Blossom and twig blight symptoms showed up at the end of May and were mostly due to Phomopsis infection during wet conditions at bloom. However, levels stayed low to moderate and did not increase much, while twig blight lesions did increase in length over time. Some Pseudomonas (bacterial) twig blight was observed on ‘Elliott’ plants. Pseudomonas blight is favored by freezing temperatures and moisture during bloom, and looks similar to Phomopsis blight except that the color of the necrotic tissue is darker, sometimes almost black. Phomopsis canker became quite apparent in many blueberry fields later in the season during and after the harvest, due to stress on the canes. Those infections are often the result of damage to the canes such as bark being scraped off the canes by mechanical harvesting the previous year. Anthracnose canker, caused by Colletotrichum acutatum was observed on canes in some fields, mostly cv. Jersey. Anthracnose lesions are more sharply delineated than Phomopsis lesions and often but not always centered on leaf scars and have small blister-like fruiting structures in expanding rings around the center of the lesion. Due to warm, humid conditions, anthracnose fruit rot incidence was high this year and affected fruit quality. Botrytis fruit rot was more common than Alternaria fruit rot as a post-harvest disease. Botrytis is characterized by fluffy white to tan-gray mycelium and tan to gray powdery spore masses. Some frost damage was also observed on fruit as well as leaves (blistering on lower leaf surface along veins). Furthermore, red spots on the upper leaf surface and edema (tiny watersoaked spots on lower leaf surface) were observed on leaves in many locations and were probably caused by spray injury or environmental conditions. Powdery mildew was common later in the season due to warm humid conditions and was characterized by yellow to reddish blotches and mild wrinkling of leaves. Leaf rust was also observed in multiple locations; this disease if favored by warm, wet weather in mid summer.

This year, Armillaria root rot was diagnosed in several relatively young fields that were planted in areas cleared of mixed forest containing oak trees. Symptoms affected scattered bushes throughout the planting with stunted growth, leaf discoloration, root and crown rot, white fungal mats under bark, mushroom-like smell of the crown, and black, root-like strands (rhizomorphs) on the crown. In general, virus symptoms were present but not as obvious as in previous years, due to To date, a total of 28,650 leaf samples have been tested from 644 blueberry fields on 133 Michigan farms. The survey resulted in seven detections of blueberry scorch in three different areas of the state and no detections of blueberry shock. A virus-update meeting for blueberry growers in planned for late October.

Grapes

Due to plenty of moisture and conducive temperatures in the spring and early summer, Phomopsis and black rot came in very early and disease pressure was moderate to severe in unsprayed vineyards. Many growers who had lost most of the crop due to the spring freeze also applied fewer fungicide sprays and may see more disease this year.

Downy mildew also got an early start and was moderate to severe in vineyards with no or limited fungicide application. Regular rain events in the spring and early summer encouraged infection. However, dry weather in July and August slowed down the epidemic. Where disease was severe, defoliation started early. Powdery mildew showed up relatively late in most vineyards (but earlier than 2009), and some cases of berry infection were reported in ‘Concords’. However, in some wine grape vineyards, symptoms were severe and may be the result of fungicide-resistant powdery mildew strains. In most locations, powdery mildew pressure became heavier in July and August due to the dry, warm weather.

Botrytis symptoms showed up early, but in many cases were due to grape berry moth infestation of clusters. Cool wet weather during bloom may have been conducive to latent cluster infections as well. While sour rot and Botrytis bunch rot were kept at bay by dry conditions during August and early September, recent rains may change the situation rapidly. Be on the lookout for bunch rots now. A very good management approach is to remove leaves around the clusters to improve airflow and sun exposure. However, this is best done earlier in the season (between fruit set and veraison).

A less-known disease of grapes in Michigan, anthracnose, caused by the fungus Elsinoe ampelina, was again observed at multiple sites and tended to be more severe this year than last year. It was promoted warm, rainy weather in late spring and early summer. The fungus primarily attacks table grapes, but Vidal, Frontenac, and Marquette are also quite susceptible. Symptoms on the shoots somewhat resemble those of Phomopsis, but are typically more sunken with raised edges. On leaves, the center of older lesions drops out, giving the lesions a “shot hole” appearance. Lesions on green berries are purplish or grayish with darker margins, and do not expand much upon ripening – berries crack rather than rot. The fungus overwinters in infected canes. Be on the lookout for this disease and make sure to prune out infected canes.

Various leaf symptoms resembling leaf roll virus were seen in vineyards but due to the warm weather, the symptoms were not nearly as common or striking as in 2009. While grapevine leafroll associated virus 3 was detected in several locations, red leaves sometimes seemed to be associated with crowngall or some other stress on the vine. Eutypa dieback symptoms were seen in some vineyards. In early summer, widespread phytotoxicity was seen due to the new fungicide Revus Top, mostly on Concord and Noiret vines. Use of this fungicide was halted immediately. The exact reason for the phytotoxicity was not discovered, but may have had something to do with tankmixes with foliar fertilizers or specific adjuvants and the tender state of new grape tissues growing rapidly during warm weather after a cold spell.

Strawberries, brambles, and saskatoonberries

Wet conditions in late spring and early summer favored foliar and cane diseases in brambles, and particularly Botrytis gray mold on the fruit. Spur blight started to appear in early June and increased rapidly for a 4-6 weeks. Leaf spot did not become common until later in the season. In tunnel production, powdery mildew and even late leaf rust became problems later in the season. Botrytis cane blight can also be a problem in greenhouses. I have noticed that thornless blackberries suffer from various cane blights, which seem to be secondary to winter injury.

In strawberries, foliar disease pressure was moderate, but wet conditions during fruit ripening did lead to fairly high leather rot (Phytophthora cactorum) pressure. In one case, poor establishment in a young strawberry field was related to black root rot caused by Cylindrocarpon, Pythium, and Rhizoctonia spp. In another case, poor growth and stunting, which appeared like black root rot symptoms was caused by cyclamen mite.

In a saskatoonberry field in northern Michigan, leaf rust was a problem in 2009 and was identified as Gymnosporangium nelsonii. This fungus overwinters on junipers. A disease control program using sterol inhibitor fungicides effectively controlled this disease this year. Entomosporium leaf spot, caused by Entomosporium mespili showed up in August and could lead to premature defoliation.

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