Bacterial Diseases in Tree Fruits

Gordon Johnson, Extension Vegetable & Fruit Specialist;

This is the time of spring when we see the results from fire blight infections in pome fruits. In addition, warm weather with higher humidity and more frequent showers creates conditions that are very favorable for bacterial spot in stone fruits.

Fire Blight in Pome Fruits
May is when fire blight peaks in apples, pears, Asian pears, and ornamentals such as crabapples and flowering pears from earlier flower infections. After bloom, shoot blights are common in new growth. We are seeing both currently.

The fire blight pathogen, Erwinia amylovora, overwinters on branch cankers from the previous year’s infections. In spring, as temperatures warm, bacteria multiply at the edge of these cankers and create a yellow exudate that oozes on the bark surface several weeks ahead of bloom. Prior to bloom, insects that are attracted to the ooze, such as flies, spread the bacteria throughout the orchard. During bloom, pollinating insects (bees) spread the bacteria to the blooms. Blooms are susceptible to infection up to petal fall. Infections occur when temperature and moisture conditions are favorable, that is greater than 60°F with free water (rain or dew). Infection symptoms will appear 1-4 weeks after bloom. In addition, shoot blights can occur when inoculum is high in the orchard. Shoot tip infections occur most commonly on watersprouts and young shoots with about 10 leaves.

Fire blight shoot strike.

Fire blight shoot strike.

Symptoms of the blossom blight phase of fire blight will be the wilting and death of flower clusters which then can spread to the branch and kill portions of the branch. Areas turn dark in color (brown or black). Shoot infections appear as a wilt with a characteristic “shepherd’s crook” symptom. Shoot infections can also spread to nearby branches and even the main trunk. Fire blight infected areas are often called “strikes”. There are apple rootstocks that are highly susceptible to fire blight (M.26, M.9, Mark). If they become infected, the canker will infect the trunk of the rootstock below the graft union and the tree will decline over 1-2 year period. What makes this disease particularly devastating is that one flower or shoot infection has the potential to kill the whole tree (particularly in young orchards).

In fire blight susceptible orchards, prebloom sprays of copper fungicides can help reduce the bacteria on plant surfaces. Use bloom sprays of the antibiotic Streptomycin on a 3-7 day schedule when conditions are favorable (above 60° F, and >60% humidity). Post bloom Streptomycin sprays may also be needed with susceptible trees to control shoot blights.

According to Penn State, post-bloom, to prevent shoot blight, prohexadione calcium (ProCa; Apogee/Kudos) applications should be used. Depending on the size of the tree (and rootstock), 2 – 12 oz/A is recommended. It takes 10-14 days for ProCa to harden off shoots, which makes the fire blight bacteria, Erwinia amylovora, less likely to invade shoots and causing shoot blight. Repeated applications may be necessary. Regular Cueva (type of copper) applications (2 qt/A) has shown to limit shoot blight when low to moderate disease conditions occur.

There are fire blight resistant apple and pear varieties and rootstocks. In our Delmarva production area, growers should consider using resistant varieties if they meet market and quality standards rather than trying to control the disease with sprays. Fire blight resistant apple rootstocks are also advised for our area.

Once fire blight “strikes” occur on branches, there is no curative action that can be taken. These strikes must be pruned out below the strike (8 inches below the visible discolored branch area) and destroyed. Do not leave the blighted prunings in the orchard. Also disinfest pruning shears and loppers between cuts using alcohol or bleach solutions to avoid inadvertent spread. If main trunks are infected, they should be cut 8 inches below the visible infection.

Bacterial Spot of Stone Fruits

Bacterial spot on nectarine leaf. Note shot hole appearance

Bacterial spot on nectarine leaf. Note shot hole appearance

Current weather conditions are favorable for the development of bacterial spot in susceptible stone fruits including peaches, nectarines, apricots, plums, pluots, apriums, and plumcots. Bacterial spot is caused by the organism Xanthomonas campestris pv. pruni. It is found on leaves, twigs, and fruits. This time of year, we see the results of spring twig infections with shoot tips that are dead. Leaf symptoms after infection are most readily seen as a “shot hole” appearance, where the small, infected areas dry up and fall out, or as tattered leaf edges. Fruit infections are dark colored small spots on fruit skin in that then lead to fruit cracking later as spots coalesce.

The bacteria overwinter in twigs that were infected in the previous fall from diseased leaves. In the spring, during warm, wet conditions, the bacteria ooze out and can be splashed onto leaves and fruit. Fruit and leaf infections start around shuck split and then can continue throughout the season in susceptible varieties. Infections only occur during wet conditions.

In bacterial spot susceptible varieties, sprays of copper fungicides are applied in early spring prior to bloom to reduce surface bacteria numbers. To control the disease during the season, sprays should be applied from petal fall until 2 weeks before harvest. In wet conditions, applications should be close together (5-7 days), in dry conditions, applications can be spread further apart. Use antibiotic products (Mycoshield, Fireline) or use fixed coppers with low phytotoxicity potential. Copper can cause leaf damage so care should be taken with their use.

The best management strategy for bacterial spot is to use resistant varieties. Many eastern bred varieties have good bacterial spot resistance. Western bred varieties, developed in lower humidity areas, are often very bacterial spot susceptible and will be difficult and expensive to produce on Delmarva.