The two most common problems that Minnesotans who grow blueberries are likely to encounter are iron chlorosis and winter injury. Both disorders can kill plants and limit the range where blueberries can be grown in Minnesota.
Iron chlorosis occurs when the leaves cannot produce chlorophyll due to a lack of iron in the leaves. Without chlorophyll, the leaves do not turn green. Blueberries suffering from iron deficiency are usually pale yellow to white, while the leaf veins remain green. In some cultivars of blueberries, the leaves will develop a reddish color. With iron chlorosis the youngest leaves are yellow, while older leaves will retain a normal green color. With most other nutrient deficiencies, the older leaves turn light green before the youngest leaves. Leaves with severe iron chlorosis die, and the entire plant will die if all the leaves are chlorotic.
Iron chlorosis is caused by a lack of available iron in the soil. Although iron is abundant in most soils, it becomes unavailable to plants when the pH is higher than the optimum for that plant or when soils are saturated with water. Blueberries can develop iron chlorosis when the soil pH rises above 5.5. The exact pH where blueberries will display symptoms of iron chlorosis varies between cultivars. The cultivar 'Northland' often shows iron chlorosis symptoms before other common cultivars grown in Minnesota. Plants growing in sandy soils can tolerate a higher pH than plants growing in soils that are high in clay.
Iron chlorosis can be minimized by choosing the correct site to grow blueberries and by properly amending the soil. To learn more about how to choose and amend a blueberry planting site, see 'Blueberries for Home Landscapes". In many soils, the pH will slowly rise over time, especially if the plants are irrigated with hard water from wells. Soil pH can be maintained low enough for blueberries with a combination of fertilizing with ammonium sulfate and with elemental sulfur. Elemental sulfur gives the most acidity per pound of any product, but acts too slowly to cure the symptoms of iron chlorosis during one growing season. Adding elemental sulfur every two or three years will keep the soil acidic. Oak leaves and pine needles that are used as mulch also help maintain an acidic pH.
For a temporary solution, plants can be sprayed with a foliar iron fertilizer, which are sold at many garden centers. Always make sure that the fertilizer has "chelated" iron in the ingredients. Chelated iron sprays only reduce iron chlorosis on leaves that have already formed. Leaves that form after the spray must be sprayed again.