Bottles of lemon, orange, and apple juice with a knitted beanie.

We often think of plants as helping us raise our own immunity against common viruses like cold and flu. However, plants can be infected with diseases, too. While the study of plant viruses is fairly recent, infectious plant diseases have been around for some time. Read on to learn more about common diseases that can affect your orchards and effective measures for curbing the damage.

Copper Spray — Your Number-One Ally Against Disease

Fortunately, while there are several different plant viruses that often infect fruit trees, a spray with fixed copper during the dormant period is effective against most of them. Of the main viruses that infect plants, copper can help to prevent:

  • Bacterial canker
  • Brown rot
  • Coryneum blight
  • Peach leaf curl

The way that suspended copper particles work to prevent plant viruses is by getting in and killing the spores before they have a chance to enter the plant. Once a plant is already infected with a disease, copper sprays are not as effective. 

When choosing a copper spray to use in your orchard, be sure to check the concentration of copper and the specific instructions for the spray that you choose. For example, spraying too much copper, too late could cause russeting on apples and pears. As a general guide, the University of California recommends a solution that contains 50% copper for application during the dormant season. 

The Main Plant Viruses That Can Affect Your Orchard

Now, we’ll go ahead and list the main viruses that infect plants, along with symptoms, prevention, and options for treatment.

Peach Leaf Curl

Peach leaf curl is caused by the Taphrina deformans fungus and affects peach and nectarine trees in cool, wet climates. Peach leaf curl shows up as red-colored lesions on the fruit along with red-colored curling on the leaves.

To prevent peach leaf curl, spray the tree with an appropriate copper solution just after leaf drop, again around January 1, and a third time before bud swell. Once peach leaf curl is already active, thin the fruit to reduce the burden on the tree’s resources and clear up any fallen leaves and fruit to prevent the disease from spreading to other trees. Leaves and fruit affected by plant viruses should always be discarded — not composted — as the spores can lay dormant and could reinfect your orchard next year.

Apple and Pear Scab

As the name suggests, this fungal springtime disease affects apple and pear trees, leading to damaged leaves and dark scabby patches on the fruit. Generally, apple and pear scab infects trees during wet conditions from the green tip stage through to the blooming period.

To stop the spread of plant viruses like scab, remove any fallen leaves and discard them rather than adding them to the compost. If rust is a common problem in your area, or your local agricultural extension office forecasts an outbreak of scab based on local moisture and temperature conditions, you can spray your trees at the green tip stage with fixed copper, micronized sulfur, or lime sulfur — always following the directions on the label.

Bacterial Blossom Blast

Bacterial blossom blast is caused by a bacterium called Pseudomonas syringae that can infect fruit buds if freezing-cold temperatures occur during bloom. This disease affects stone fruits and pear, and “blasts” the buds, blossoms, leaves, and developing fruit. If you know that freezing temperatures are likely to occur during blossoming, cover your fruit trees with blanket bags or frost shields to prevent blossom blast and other plant viruses. Orchardists who have suffered from blossom blast in a previous season can apply fixed copper preventatively from green tip through to bloom.

Fruit Brown Rot

Fruit brown rot is very similar to blossom blast, except that it is caused by a fungus rather than a bacteria. Targeting stone fruit and almond trees, brown rot spreads through gray spores and blights blossoms, leaves, twigs, and fruit. To control brown rot, remove and discard any fruit and wood that has been affected. Micronized sulfur and fungicidal copper spray can be applied preventatively if this fungal infection or other viruses that infect plants have been a problem for your orchard in the past.

Fire Blight

Fire blight is very similar to blossom blast in that it blackens the leaves and stems. However, this disease becomes active under wet conditions with an average temperature above 60°F as opposed to freezing cold weather. Fire blight is caused by the Erwinia amylovora bacteria and principally affects pear, apple, and quince trees. To prevent further spread — as with other plant viruses, cut out the infected material well before injury is visible. If needed, you can also apply a preparation containing the beneficial bacteria Bacillus pumilus, which acts as an organic fungicide.

Bacterial Canker

Just like plant viruses such as bacterial blossom blast, bacterial canker is caused by the Pseudomonas syringae. However, while blossom blast affects stone fruits and pear trees, bacterial canker affects stone fruits and almonds. When a tree becomes infected with bacterial canker, a canker develops on the branch at the place of infection, causing the branch to girdle and a dark-colored gum to seep through the bark. As a result of the girdling of the branches, the foliage collapses and dies.

Generally, this bacterial disease dies off when the weather becomes warmer and drier. In the meantime, keep the trees strong and healthy through good cultural practice. You can also help to prevent bacterial canker by applying certain agricultural products when replanting your orchard.

Nip Plant Viruses in the Bud with Fruit Growers Supply

Plant viruses can affect buds and blossoms.

After reading about all of the viruses that infect plants, take a proactive step by stocking up on products and talking to an expert about preventing disease for the upcoming growing season. At Fruit Growers Supply Company, we’ve been working with California growers since 1907 and we stock the solutions that really work. Give us a call or drop into one of our full-service stores in Orange Cove, Woodlake, Riverside, Porterville, or Santa Paula to prepare for a healthy, disease-free harvest.

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