TREES - FRUIT

Apple scab
Fire blight


Apple Scab
Apple scab is a fungal disease that occurs most commonly during cool, moist summers. In other areas such as the northeastern United States, apple scab is much more common and more severe than in Montana. The disease is more common in western Montana than in eastern Montana.
 
Fungus: Venturia inequalis
 
Hosts
Apple and flowering crabapple are susceptible to scab. The fungus that causes pear scab is closely related to the apple scab fungus, but does not infect apple or crabapple and vice versa..
 
Symptoms:
Symptoms occur on both the leaves and the fruit. Look for leaf spots that are pale yellow when young, but then become olive-green and then nearly black in color. The margins of these spots are feathery and indistinct. The spots can darken to nearly black. Severely-infected leaves may curl and crack. On fruit, small, dark, raised areas appear first. Later, the skin ruptures and the exposed fruit tissue has a brown or black velvety appearance. As these spots enlarge and age, the center turns brown and corky. Fruit becomes distorted and young fruit drops.
 
Disease cycle and favorable conditions:
The disease spores spend the winter in fallen leaves and in the soil. In early spring, wind blows the spores to new host trees. During damp periods, new leaves become infected easily. The longer the leaves are wet, the more severe the infection will be. Once the disease has infected the tree, damage to the crop may be minimized but not cured.
 
Controls:
When the weather is favorable or there have been past problems with apple scab, preventative spray programs must begin as soon as leaf growth appears. This is because spores readily infect leaves from the time of cluster bud break until the leaves are fully expanded. Preventative spray programs entail regular application of a fungicide throughout the season. Fungicides such as captan, Funginex, mancozeb, and wettable sulfur are effective. When using any pesticide, whether it is natural or synthetic, be sure to read and follow label instructions carefully to ensure personal safety, and safety to others and the environment.
 
Resistant varieties:
A much easier way to deal with apple scab is to plant scab resistant varieties. There are numerous crabapples and apples that are resistant to apple scab and need no spraying for disease control.
 
Crabapple Varieties - Susceptible to Apple Scab
 Almey, Hopa, Red Silver Van Eseltine, American Beauty, Pink Perfection, Royal Ruby, Vanguard,Barbara Ann, Radiant Royalty, Dorothea, Red Jade, Spring Snow
 
Crabapple varieties - Good to excellent resistance to apple scab
Adams, Harvest Gold, Robinson, Bob White, Liset, Sentinel,Centurion, Madonna, Snowdrift, Donald Wyman, Molten Lava Sugar Tyme, Christmas Holly, Prairifire

Source: Apple Scab and Flowering Crabapples, http://www.ipm.iastate.edu/ipm/hortnews/1991/6-5-1991/scab.html Richard Jauron, Department of Horticulture, Iowa State University, Ames, Iowa.)
 
The following is a list of some apple cultivars for western Montana. Only cultivars rated as "very hardy" are recommended for eastern Montana. The list is adapted from ratings compiled by Nancy Callan (Western Agricultural Research Center, Corvallis).
 
FB* RUST SCAB MILDEW HARDY

RIPENS AUGUST:
Lodi S - - - -
Geneva Early S S S - -
Quinte - - - - H
Julyred S - S - H
Jerseymac S - S - -
State Fair S - - - H
Redfree MR R R - -
 
RIPENS SEPT.:
Wealthy S - - - H
Prima MR R R R -
Novamac R R R S -
Chehalis - - R - -
Gala S - - - T
McIntosh MR - S - H
 
RIPENS OCT:
Sweet Sixteen MS - - - H
Macoun - - - - H
Spartan R - R - -
Cortland - - - - H
Delicious - - - - -
Empire - - - - -
Priscilla R R R S T
Johnathan - - - S T
Johnafree R R R S -
Honeycrisp - - - - H
Nova Easygro R R R R -
SirPrize S S R R -
MacFree R R R S -
Liberty MR R R S -
Freedom R R R R -
*FB = Fire Blight
 
More on apple scab - make LINK to Cornell University Apple Scab Home Page
http://ppathw3.cals.cornell.edu/profiles/applescab/pa2200t0.html

References:
Flint, M.L. 1990. Pests of the Garden and Small Farm. Univ. of California. Oakland, CA 94608- 1239. Pub. #3332. 276 pp.

Gilkeson, L., and P. Michalak. 1994. Controlling Pests and Diseases. Rodale Press, Emmaus, PA 18098. 159 pp.

Pacific Northwest Plant Disease Control Handbooks. 1997. Extension Services of OSU. Corvallis, OR 97331-0817.

Pest Alert, SDSU Cooperative Extension Service, July 15, 1995, Vol. 4 No. 7
 
Written by Sherry Lajeunesse and Martha Mikkelson, Extension Urban Pest Management Specialist and Plant Disease Diagnostician, 1998.
 

Fire Blight
 
Cause
Fire blight is caused by the bacterium Erwinia amylovora.
 
Hosts
Fire blight attacks many trees and shrubs in the Rose family. These include apple, crabapple, pear, mountainash, flowering almond, hawthorn, and cotoneaster during wet weather. Raspberry, rose, serviceberry (saskatoon), and spirea sometimes are infected as well. However, the strain of bacterium that infects raspberry does not infect apple and vice versa.
 
Origins of fire blight
Fire blight is native to North America and likely occurred on native plants such as crabapple, hawthorn, and mountain ash. The disease probably spread to susceptible cultivated apples, pears, and woody ornamentals planted by the early American pioneers. As the settlers moved west, so did fire blight. By the early 1900's, fire blight was a serious threat wherever apples and pears were grown.

It is interesting to note that stringent quarantine laws in countries such as Germany have been largely successful in keeping the disease rare in Europe. Also of interest is this little-known fact: In 1885, Joseph Arthur proved that fire blight was caused by a bacterium and for this he earned the first Doctor of Science degree awarded in America.
 
Symptoms
Symptoms include blossoms and young leaves that suddenly turn brown and wilt. Later, the twigs may appear scorched by fire, and may curl into a characteristic "shepherd's crook". Severe infections ultimately can be fatal. The leaves on infected branches often remain on the trees all summer and well into the winter. Fire blight also produces cankers on branches and twigs. These cankers consist of discolored areas that are slightly sunken and tend to crack around the edges.
 
Disease spread
The cankers may produce a slimy, bacterial ooze that contains millions of bacterial cells. Flies, bees, ants, aphids, and beetles can spread these bacteria from blossom to blossom and from tree to tree. The disease also can spread by rain, wind and pruning shears.

Most fire blight bacteria survive from one season to the next in hold-over cankers on infected trees. Infections occurring during bloom and during the prolonged cool, moist periods mostly come from the bacteria in the cankers. Other minor sources include hail, insects, and wind.
 
Favorable conditions
Hope for dry weather during blossom time. Higher temperature and lack of free moisture on the blossoms stops the activity of the bacteria. Fire blight most commonly develops in the late spring to early summer. Temperatures between 65 F and 86 F and plentiful rain with alternating periods of sunshine are necessary for disease to occur.
 
Economic damage
Fire blight is the most damaging disease of apples and pears in North America. Losses from the disease cost growers millions of dollars annually. Fire blight damages the current years crop as well as causes permanent damage to orchards.
 
Cultural control
Avoid heavy, early-season fertilization that results in profuse succulent new growth. It is especially important to keep watersprouts (new shoots growing from the base of tree) pruned. Watersprouts are very susceptible to the bacteria and infection can progress rapidly into the tree base causing loss of the whole tree.

Once fire blight has infected a tree or shrub, eradication is the only way to control it. During the dormant period, prune out the infected plant material, cutting at least six inches below the diseased wood. Disinfect pruners between each cut by dipping them in a solution of one part household bleach to nine parts water for ten seconds. Burn the diseased prunings.

Prevent problems by planting resistant varieties, or spray existing plants with an antibiotic such as Agristrep, every five to ten days during bloom. The addition of Regulaid as an adjuvant will increase absorption of the streptomycin. Excessive fertilization, especially with nitrogen, can make plants susceptible to fire blight. Fertilize only when soil tests show a nutrient deficiency. Rapid growth can make the tree or shrub susceptible to other problems as well; steady growth is best.
 
Control with Antibiotics
In very early spring, spray with a dormant oil plus copper-based fungicide before the buds begin to swell. Antibiotic sprays are only effective during flowering and in the summer. Mid-season sprays generally are not cost effective except after hail storms or other damage occurs.

When properly timed, antibiotic applications during bloom are effective. However, rain and temperatures between 65 F and 86 F are necessary for infection and disease spread. If blossom infection is well controlled, fire blight is often not a significant problem later.

Streptomycin (Agristrep) is the most effective antibiotic, except in areas where streptomycin-resistant strains of the bacterium have developed. Where streptomycin is not effective, copper-based bactericides, oxytetracycline, and flumequine are possible alternatives. Because of their limited systemic and curative action, bactericides are applied as protectants two or three times during the bloom period. Apply more often when it rains!

Repeated use of all antibiotic materials could lead to antibiotic resistance in fire blight bacteria. Resistance to both antibiotics and copper compounds has been found in fire blight bacterial populations in Washington and elsewhere. Resistance is not known to occur in Montana yet.
 
Biological control
Biological control of fire blight also is under study. Applications of antagonistic bacteria such as Pseudomonas fluorescens strain A506 and Erwinia herbicola strain C9 have been fairly effective in Oregon-based studies. Plant Health Technologies, a subsidiary of J.R. Simplot Co., is evaluating these organisms and plans to pursue EPA approval if results continue to be good. Development and approval processes move slowly, however, and a commercial biocontrol product probably won't be available before 1996 or later (check into this!)
 
Resistant varieties
Apple breeding programs have provided new cultivars with greater resistance to a number of diseases, including fire blight, apple scab, powdery mildew, and rust.
 
The following is a list of some apple cultivars for western Montana. Only cultivars rated as "very hardy" are recommended for eastern Montana. The list is adapted from ratings compiled by Nancy Callan (Western Agricultural Research Center, Corvallis).
 
FB* RUST SCAB MILDEW HARDY
RIPENS AUGUST:
Lodi S - - - -
Geneva Early S S S - -
Quinte - - - - H
Julyred S - S - H
Jerseymac S - S - -
State Fair S - - - H
Redfree MR R R - -
 
RIPENS SEPT.:
Wealthy S - - - H
Prima MR R R R -
Novamac R R R S -
Chehalis - - R - -
Gala S - - - T
McIntosh MR - S - H
 
RIPENS OCT:
Sweet Sixteen MS - - - H
Macoun - - - - H
Spartan R - R - -
Cortland - - - - H
Delicious - - - - -
Empire - - - - -
Priscilla R R R S T
Johnathan - - - S T
Johnafree R R R S -
Honeycrisp - - - - H
Nova Easygro R R R R -
SirPrize S S R R -
MacFree R R R S -
Liberty MR R R S -
Freedom R R R R -
*FB = Fire Blight
 
References
 Brown, Bill. Pest Alert. 1995. Colorado State University, Vol. 12, No. 1.

Brown, Bill. Pest Alert. 1995. Colorado State University, Vol. 12, No. 4.

Brown, Bill. Pest Alert. 1995. Colorado State University, Vol. 12, No. 5.

Pacific Northwest Plant Disease Control Handbooks. 1997. Extension Services of Oregon State University. Corvallis, OR 97331-0817.

Pests of Landscape Trees and Shrubs; An Integrated Pest Management Guide. 1994. University of California. Oakland, CA 94608-1239. Pub. #3359. 327 pp.

Plant Health Guide. 1995. Meister Pub. Co. Willoughby, OH 44094-5992. 178 pp.

Sinclair, W., et al. 1987. Diseases of Trees and Shrubs. Cornell Univ. Press, Ithaca, N.Y. 576 pp.

Watkins, J.E. 1992. Fire blight of apple, pear, and woody ornamentals. NebGuide, G92-1120-A.

Written by Sherry Lajeunesse and Martha Mikkelson, Extension Urban Pest Management Specialist and Plant Disease Diagnostician. January 1998.