I have a Santa Rosa plum ... #647034 - Ask Extension


I have a Santa Rosa plum ... #647034

Asked June 06, 2020, 2:54 PM EDT

I have a Santa Rosa plum tree that was very productive last year. This year there was an abundance of blossoms and early developing fruit in about mid-may. All of a sudden, the blossoms and stems turned brown and fell off the branches. (1) what causes this? (2) what can I do to prevent this from happening again? (3) do I need to spray before the blossoms appear and if so, what is the best spray to use? Thank you, Leighton Hazlehurst Hood River

Hood River County Oregon

Expert Response

It is difficult to say what the exact culprit may be without knowing your exact horticultural practices (watering, fertilizing, pruning) and seeing the number of blossoms or fruit that fell off the tree. 

This sounds potentially weather related. Trees often drop small fruit and blossoms when weather becomes warm and sunny in May and June. This is due to a carbohydrate deficit in the tree. Typically, you will loose the smaller, weaker fruit and blossoms. 

Loss of blossoms may also be due to a sudden cold spell. 

In some instances blossom loss will be due to a bacterial infection called blossom blast, which usually occurs in conjunction with other symptoms such as cankers and oozing tree sap. 

Ashley Thompson Replied June 10, 2020, 8:11 PM EDT
Thank you for your reply.
I have just recently read on google something called brown rot or blossom rot which can occur with stone fruit, like my Santa Rosa plum tree. Some of the responses to my google search also say it is related to weather changes, but all describe a condition which I witnessed on my plum tree; namely, brown flowers and stems fall off the branches before any significant fruit forms.  The blossoms and stems seem to fall off almost completely over a period of  just several days
I don't like to spray the tree, but it is suggested that the tree needs to be sprayed with a fungicide every spring just after the tree begins to bud.  Among the treatments recommended are lime sulfur and captan.
Would appreciate your thoughts on treatments.
Thank you again, Leighton Hazlehurst

The Question Asker Replied June 10, 2020, 8:59 PM EDT
Plant diseases require the correct conditions and pathogens to infect the plant. Brown rot is typically a disease of warm wet climates and we do not usually experience the necessary conditions for infection East of the Cascades. Did you notice any mummified fruit around your tree from last year? Did you have any fruit with brown rot last year. These mummified fruit would be necessary to produce spores to land on the flowers during a warm wetting period. Dormant sprays can be used to control brown rot. As previously mentioned, it is difficult to tell you what is happening to your tree without any pictures or information about your practices. Refer to https://catalog.extension.oregonstate.edu/ec631/html to develop a spray program for your plum tree.
Ashley Thompson Replied June 10, 2020, 9:30 PM EDT

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