Gummosis of Stone Fruits
June 2, 2011
In this Issue:
What to Look for/Do Now:
Current Insect and Disease Activity
Because of the cool, wet spring, codling moth emergence has been very sporadic. Biofix (first trap catch) is about 2 weeks later than last year in most locations. Adults only fly at night, looking for mates and egg-laying sites, when temperatures are above 50 and when there is not a pouring rain. The weather has surely prevented mating, and thus egg-laying from happening so far. Eggs within females are only viable for about 3-4 days, so if they cannot mate with males, eggs do not get laid. We are hoping that this means a “light” population for the first generation flight. Updated spray timings are posted below.
Peach twig borers spend the winter as small larvae in galleries within the tree cambium. Larvae have emerged and moved to newly expanding shoots where they bore into the tip to feed, causing the typical “shoot strikes.” They will pupate within the shoots and emerge soon as adult moths, to lay eggs on shoots. Later generations feed on the fruit. We have not caught any moths, and when we do, we will be able to provide dates for starting sprays.
Aphid eggs that survived your dormant oil sprays have hatched and are multiplying now, including green peach aphid, leaf curl plum aphid, and black cherry aphid. Be sure to examine the aphid colonies closely for beneficial insects, as they are also increasing in population.
Green peach aphid is the most damaging, especially to nectarines, in that feeding can result in deformed fruit. All three species leave their tree fruit hosts for the summer to vacation on alternate plant hosts, returning at the end of the season to mate and lay eggs.
Oil (1%) or insecticidal soap are the best options for residential growers. Once leaves are tightly curled, aphids are more difficult to treat, so thorough coverage is important.
With the warming temperatures comes the flow of sap, especially noticeable in stone fruit trees (cherry, apricot, peach, and plum). The oozing is sometimes generically referred to as gummosis (although back East, “gummosis” refers to a disease caused by a fungal pathogen). The sticky, oozing gum can be clear or amber in color, and by the end of the summer, it will have become almost rock-hard.
1. A plant pathogen
a) Cytospora canker, caused by Leucostoma cincta, invades and kills bark and cambial tissue through wounds such as pruning cuts, sunscald, hail, etc. Gumming from cytospora is dark amber in color, and if you scrape the outer bark, the dead phloem will appear cinnamon brown in color. Cytospora canker is an opportunistic pathogen, meaning that it invades trees through wounds. It can be found almost everywhere, so prevention is the key to management.
Management for cytospora includes:
i. in normal pruning operations, make proper cuts (i.e., do not leave stubs or do not make “flat cuts” that remove the branch collar where healing would normally occur) and do not prune in wet weather;
b) Bacterial canker, caused by Pseudomonas syringae, is more common on cherry, but has been identified from peach in Utah. Gumming from bacterial canker can also be amber in color, but may appear milky. Exposed phloem will have a slight fermented smell. Pseudomonas is also opportunistic, entering the plant tissue through tiny wounds. The bacteria can survive as a non-pathogenic epiphyte on leaf and bark surfaces of peaches, cherries, and many other plants including weeds.
Infections occur in late fall just prior to winter, and symptoms appear the following spring. Late season rainfall spreads the bacteria from leaf surfaces to buds where the infections take place. Infections are inconspicuous in the fall and winter but become more obvious in the spring, with dead buds that often exhibit signs of gummosis. Flowers and foliage may be infected under rainy conditions, resulting in wilting of shoots and oozing. Once within the branches, the bacteria invades the phloem, causing cankers
Management is the same as for cytospora canker, with the addition of copper sprays in the fall (2-3 applications beginning at 10% leaf drop to just after full leaf drop) and early spring prior to bud break.
2. Greater peachtree borer.
If you see gumming at the base of the tree (no higher than 8-12”), the gumming may be caused by this borer, and is not a canker. Peachtree borers attack the crown of the tree, and healthy trees can withstand attack. Tree can be protected with a properly timed insecticide (more information in future advisories).
3. Flatheaded or shothole borers.
These types of beetles will only attack weakened trees or wounds such as where sunscald has occurred. Attacks on healthy trees are usually unsuccessful because the tree will exude enough sap/ooze to flush out the insects. Ooze is often clear in color, and limited to beetle entry holes. Weakened trees that are attacked may actually not ooze as much because they do not have enough reserves to waste on this response. Management of these pests is difficult, and may include bark sprays of permethrin May through August.
Wounds from frost crack, bark injury, cat scratching, hail, etc. may exude gum in spring. Gummosis not caused by a pathogen will run somewhat clear in color (but will dry to amber).
5. Other climatic or physiological problems
Factors such as planting too deep, excessive irrigation, severe pruning from April - August, or over-bearing have all been cited as possible causes of abiotic gummosis.
If you are not sure that a pathogen is causing the gummosis, scrape the outer bark away. If the inner bark is still cream-colored (healthy), the oozing is caused by a non-living factor, and there is nothing you should do. If the wood is tan to brown, it is dead, and was most likely killed by a pathogen.
Coryneum blight (also known as shot hole) infections on fruit are showing up now, especially on apricots. The cool wet weather has been ideal for spread of this disease. As mentioned in earlier advisories, the optimal timing for coryneum is at the shuck split stage of fruit development.
Coryneum blight is caused by a fungus that overwinters in buds, causing small gummy cankers. From there, it spreads to leaves and later, to developing fruit. Infections on the leaves cause small round holes, with the center of the lesion sometimes barely attached. On fruit, lesions vary from dark colored warts to sunken lesions (depending on time of infection). Look for developing lesions (holes in the leaves and purplish spots on fruit) and treat if necessary to protect fruit for later in the season.
Keep in mind that future infections will occur whenever there is a 4-hour window of moisture.
Management for residential growers: If your peaches are at the shuck-split stage (currently only cooler areas such as Cache County, northern Box Elder County), you can use chlorothalonil (Fertilome, Bonide Fungonil). If your fruit is beyond this stage, you cannot use chlorothalonil. The only other easily available option is captan (Bonide). If you can find it, Ziram is very effective, and if you can afford it, so is Pristine.
Management for commercial growers: Ziram and Pristine are most effective, Abound is somewhat effective.
For all growers: an application of copper at 50% leaf drop in the fall is an excellent option for control of coryneum blight.
Peach leaf curl is showing up in many places this spring due to the cool wet weather. Peach leaf curl is a fungal-caused disease that affects peach and nectarine. Infection occurs just as the leaves are opening, and causes puckering and distortion of the leaves. The affected area is pink at first, and then turns green, then brown. Leaves will eventually drop. Infections only occur when temperatures are below 79 F in the presence of moisture. Once the temperatures rise, further infections of leaves and fruit will end.
If you see these symptoms, note that there are no fungicides that can be applied at this time. The best treatment is a single application of a fixed copper applied at leaf fall.
For now, maintain tree vigor of infested trees by thinning more fruit than normal, reducing drought stress with irrigation (if soils ever dry up), and apply extra nitrogen fertilizer.
Click here for information on degree days.
March 1-May 31
The table below shows two options for the first spray of the first generation. Option A may provide a slight cost savings, and can be repeated at the beginning of the second generation. It uses horticultural oil (1%) to target eggs before they have started to hatch. The second spray will then be about 7-12 days later, and will coincide with the period when eggs would normally be rapidly hatching. Option B is the traditional date to start sprays--when the eggs start hatching.
Good residue (insecticide) coverage is important at this timing. After the first insecticide spray has been applied, continue to apply your chosen material(s) at the interval provided on the label.
The options provided below are not all-inclusive and are not endorsements of USU. Please check the label before mixing.
Note that these treatments are only recommended if you know you have the particular pest in your trees.
Precautionary Statement: Utah State University Extension and its employees are not responsible for the use, misuse, or damage caused by application or misapplication of products or information mentioned in this document. All pesticides are labeled with ingredients, instructions, and risks. The pesticide applicator is legally responsible for proper use. USU makes no endorsement of the products listed herein.