Overview of small fruit diseases during the 2007 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 growing season started out about two weeks earlier than normal, but freeze events in April damaged grapes as well as other fruit crops in many areas in Michigan. However, due to compensation by the vines, the current grape crop overall is larger than originally thought. Blueberries sustained some damage from the freeze and strawberry blossoms were frozen in some locations. Much of the 2007 growing season (June and July) was relatively dry and warm. The blueberry crop was harvested earlier than usual and some summer-fruiting raspberries produced a second crop in the fall. Drought conditions were starting to affect fruit quality as some shriveling was observed in grapes in early August. The good news was that the dry weather overall was not very conducive to diseases, therefore fewer fungicide sprays were needed and fruit quality was generally better than in previous years. However, August brought a lot of rain resulting in outbreaks of downy mildew in grapes and an increase in fruit rots, particularly sour rot in grapes.

Most fungi and bacteria need free moisture in the form of rain or dew to grow and infect plants, and often also for spore production and dispersal. The lack of precipitation definitely thwarted pathogen activities. The only fungi that were content without rain were the powdery mildews, which were quite common on numerous crops this year. Also, diseases that resulted from infections in previous years, e.g., cane diseases and virus infections, were evident but were obviously not related to the weather this year.

Even though dry weather generally suppresses diseases, extreme drought can cause plant stress and can thereby predispose them to certain diseases. It is therefore important to reduce drought stress by irrigating where possible, even after harvest. Drought-stressed plants may also be more susceptible to winter injury.

Grapes
Due to the warm, dry summer, most diseases were less of a problem in grapes than in previous years. Rainfall in spring and early summer did result in some Phomopsis pressure. Due to the spring freeze damage, some growers decided not to apply dormant or early-season sprays and in some locations, resulting infection turned out rather severe. Phomopis fruit rot in Vignoles came on earlier than normal. Black rot was also detected here and there, but overall incidence and severity were much lower than in previous years. Though June was rather dry, rain events in early and mid-June were quite instrumental in black rot infection and some growers may have missed an important spray around that time.

Powdery mildew showed up later than expected, considering that we had moderate to high relative humidity during most of the season. However, rain is needed for the initial release of ascospores to get the epidemic started, and it is possible that the lack of precipitation could have delayed or reduced spore dispersal. Powdery mildew on Concord leaves was late enough to be of little consequence to yield or fruit quality. Infections of leaves and clusters were noted in susceptible wine grapes.

Downy mildew was observed in early June in low-lying wild grapes but not in cultivated grapes. Due to heavy rains in August as well multiple dew events, there were outbreaks of downy mildew in many locations, particularly in Niagara grapes. Downy mildew was even observed in Concord grapes.

Also, due to the heavy rains in August, there was a sudden increase in fruit rots, particularly sour rot in grapes. Sour rot was also seen in Niagara grapes, and appeared to be due mostly to splitting of berries due to high pressure within the clusters, as many Niagara clusters and berries were very large. Fruit flies and picnic beetles did their part in spreading the disease, while ants and wasps broke through the skin of berries also resulting in an increase in rots.

Anthracnose appeared on some varieties that had a lot of disease last year in the form of lesions on leaves, canes, and tendrils. Some spots were seen on berries, but disease levels were generally low. Susceptible varieties are ‘Mars’ and ‘Marquis’ table grapes and ‘Vidal’ wine grapes.

Blueberries
In general, mummyberry pressure was light to moderate in 2007, depending on location. Over the last few years, inoculum levels have been fairly low; however, disease pressure seems to be building up again. Shoot strike incidence correlated well with the number of mummies on the ground. The apothecia developed early this year, probably in part due to the mild winter. Even before green tip, apothecia could be found. The first shoot strikes occurred in the second week of May, increased in the third week of May, and then decreased as most apothecia started to dry up. The number of mummified berries was moderate, with about 25 mummified fruit per bush.

Twig and blossom blight occurred in many locations in late May and early June and was mostly caused by Phomopsis vaccinii and Colletotrichum acutatum, though Monilinia vaccinii corymbosi (the mummy berry pathogen) also caused blossom blight (flower strikes). Botrytis cinerea was only occasionally isolated.

Anthracnose and Alternaria fruit rot were found in various fields affecting the fruit at low levels before harvest, and in post-harvest rot tests, the incidence was moderate to high. Virus and virus-like symptoms were obvious in a number of fields this year as well.

Strawberries
Dry conditions also resulted in low disease pressure in strawberries. Incidence of foliar diseases and fruit rots were generally low but may have increased in August following precipitation. Foliar diseases that were seen were Phomopsis leaf blight, common leaf spot and powdery mildew. Among the post-harvest fruit rots, Botrytis gray mold was predominant, but some leather rot was also found. Leather rot can be prevented by applying straw to minimize contact of the berries with soil or splashing or standing water.

In some strawberry fields, plant stunting and death was noticed and was mainly related to black root rot or nematode infestation or old age of the field. In one case, needle nematodes were present at very high levels and were very damaging to the planting; which had only been in the ground for two years but was producing very poorly. Establishment of new plantings was also very poor. Alternate crops in those fields were pumpkins and corn and these may also be hosts to needle nematodes.

Brambles
Foliar and cane disease pressure were relatively low, although they increased some following rains in August. Leaf spot and spur blight appeared fairly late in the season. Botrytis gray mold were common in summer and fall raspberries as well as in blackberries. In general, Japanese beetles were more problematic than diseases.

Fire blight was seen again this year in raspberries in a few locations. This disease is characterized by blackening of young cane tips that then bend over and form a ‘shepherd’s crook’. Single berries or entire fruit clusters can also be killed. The bacterium is a different strain from the one that causes fire blight in pears and apples.

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