What caused the blueberry winter injury of 2003?

Mark Longstroth

Like most people in Agriculture, I pay close attention to the weather. I pay particular attention to winter temperatures because I know that in Michigan, there is always the chance of winter injury. In 2003, we saw lots of cold injury and it was the result of several distinct events. Several Horticulturists in Michigan have identified different events they think are responsible for the winter injury we saw all around Michigan this year.

In Southern and Central Michigan, there was an extreme cold snap in early December as the plants were beginning to acclimate to winter conditions. Bob Tritten, in Southeast Michigan, saw extensive winter injury in some crops that resulted from very cold temperatures in early winter following a fall when plants were slow to lose leaves and acclimate to cold. We saw the same symptoms in Southwestern and Central Michigan. This freeze killed young trees and injured the trunks of grapevines in Southern Michigan in low spots. I think this event could have caused some injury to blueberries, especially young plants that were actively growing into November.

Over much of Michigan we had a long cold winter with very little variation in temperature in January and early February. This froze the ground under the snow cover and pine trees and other evergreens were unable to restore the water they lost during sunny and windy days. Dr. Bert Cregg of the MSU Horticulture and Forestry departments has identified this event as causing statewide injury to evergreens in the state. We saw this injury throughout the state. Dr. Cregg salt injury on plants growing near major highways where the salt used to melt ice and snow on the roads has damaged the plants nearby when the salt spray settles on the plants. We saw salt injury on peaches and blueberries in Southwestern Michigan. Dr. Carlos Garcia studied salt injury on blueberries last year and this year in Allegan and Ottawa counties and has found that the injury falls off as you move away from the road. This salt injury can be seen throughout Michigan. If you saw symptoms away from major roadways it is not salt damage.

In mid February we had a thaw with temperature above freezing for several days. In Southern Michigan we lost our snow cover and the ground thawed out. Dr Cregg has identified this event as another major cause of winter injury to many ornamental plants. Many of the perennials grown in Michigan would have completed rest by then and by late February and early March thaws followed by cold weather could have resulted in injury. In southern Michigan mulberry trees showed lots of injury everywhere I went. I think the midwinter thaw and the following cold snap caused this injury. I also think this thaw combined with the drought conditions we had last summer is responsible for most of the winter injury we see in blueberries. Early in the season there was no damage to blueberries, so I discount the early December cold snap as the reason why we saw so much injury in 2003. By the time the buds on the bushes started to swell you could see that many of last years shoots had died. Another hint is that samples taken during the winter for the salt damage survey showed no injury until mid February. To me this is a strong indication that the injury was caused during mid winter. At first, I was sure it was the early frost, but now I am convinced that it was the midwinter thaw.

Winter injury in Michigan is a common occurrence. Although growers may think that winter injury is caused by extreme cold, most winter injury is caused by cold snaps early or late in the dormant season or mid winter thaws followed quickly by cold temperatures. The injury is caused the rapid change in temperature over a short time period. The plant loses winter cold hardness and cold temperatures return more quickly than the plant can reacclimate to the cold. I am convinced that the mid winter cold snap was a triggering event. I am also convinced that there were two other contributing factors.

One important factor was the weather last summer in fact for the previous two summers. The summers have been hot and dry. Blueberry plants grow their shoots and fruit at the same time in the spring and as the fruit begins to ripen shoot growth slows and stops. As the leaves mature they get thick, waxy and stiff. I have never seen a blueberry plant wilt. When plants are extremely water-stressed their leaves scorch, drying up from the edges and in extreme cases fall off. I am sure that most Michigan Blueberry plantings have been moisture stressed for the last several falls. Plants that are stressed during the growing season are less able to withstand the cold temperatures of winter. Stressed plants acclimate to winter more slowly and cannot withstand cold temperatures as well as non-stressed plants that have accumulated lots of carbohydrates during the growing season. I think that most of the blueberry plantings were predisposed to winter injury going into the winter.

The second factor is phomopsis. Dr Annemiek Schilder thought early on that a lot of the apparent winter injury was actually phomopsis twig blight and cane canker. After looking at hundreds of blueberry plants this spring I agree that a significant portion of what appears to be winter injury is phomopsis. In most cases where the apparent winter injury was really bad, with whole canes collapsing there was almost always a canker at the base of the dead cane. Often new shoots were sprouting from just below the canker. Inspecting the crowns of severely affected blueberry bushes, I almost always found dead shoot tips among last year's new shoots. Normally after a cold winter we see lots of phomopsis canker in the summer as harvest approaches and the plant is under stress from heat and the demands of the ripening fruit. In 2003 I think we saw a lot of the cankers very early on as the weakened winter injured canes succumbed to phomopsis at the beginning of the season.

The cause of all the injury in blueberries in 2003 was a hot dry summer that predisposed plants to winter injury and disease. The extreme cold snap early in December caused little damage but the mid winter thaw followed by cold temperatures killed some tissues and weaken others. This injured and weak tissue was unable to resist the invasion by phomopsis as the spring brought warm temperatures that were ideal for the spread of the disease. This event highlights the importance of irrigation in blueberries. A mature blueberry planting can use as much as 2 inches of water a week during shoot growth and fruit ripening. Once the soil dries out both shoot and fruit growth stop. One consequence is that fruit size during later pickings will be small. As long as the soil moisture is inadequate to maintain growth the plant will suffer and the results can been seen the next year. In 2003, we had an expensive lesson on the consequences of a dry summer and fluctuating winter temperatures.