Almond orchards are treated at least once during bloom for fungal disease brown rot. Rightfully so, as the brown rot fungi (Monilinia laxa) can kill the flower, fruiting spurs, and shoots. Flowers are susceptible from pink bud to petal fall, and are most susceptible when flowers are fully open. The fungus is able to infect all parts of the flower: anthers, pistils, petals, and stigmas (Figure 1). Upon killing the flower, it can move into and kill woodier tissues of the tree. Once in the woodier tissue, it forms a canker (Figure 2). This canker can enlarge to kill the branch and also serves as the survival structure for the fungus. These cankers are the reason why it takes many years to “clean up” an orchard after a severe brown rot infection. Brown rot can also infect the jacket of the almond and kill the young fruit as well. This is called jacket rot.
Figure 1: Blossom blight of almond caused by the brown rot fungus Monilinia laxa.
All varieties are susceptible to brown rot, but they vary in their degree of susceptibility. Butte is the most susceptible variety followed by Carmel. Ne Plus Ultra and Mission are moderately susceptible, while Nonpareil and Peerless are the least susceptible to brown rot. Brown Rot Blossom blight is usually controlled with a pink bud and full bloom treatment. Many of Brent Holtz’s (UCCE San Joaquin) trials suggest that the full bloom treatment may be the most important. A third petal fall spray may be necessary in years favorable to disease (rain). If bloom is strung out and the weather is wet and rainy, no more than ten days should elapse between treatments.
A few growers have told me that they do not spray for brown rot. This may be okay if good conditions persist through bloom AND you have varieties that are not very susceptible to infection (i.e. Nonpareil). It is important to note that not spraying can lead to a serious disease epidemic: A conventional Butte orchard that missed a full bloom spray had over 50% brown rot blossom infection despite no rain!
Figure 2: Twig canker of almond caused by the brown rot fungus Monilinia laxa.
Varieties that are susceptible to Green Rot or Jacket Rot (caused by Monilina laxa, Botrytis cinerea, Sclerotinia sclerotiorum) are Butte, Ne Plus Ultra, Merced, Carmel, Price or any variety with tight clusters. Nonpareil can be affected by this disease if the right environmental conditions occur. The time of infection for Green Fruit Rot or Jacket Rot is from flower opening to petal fall.
There are several fungicides that provide excellent control of brown rot. These include the fungicide classes of Strobilurin/Boscalid (i.e. Pristine), Benzimidazole (i.e. Topsin/Benlate), dicarboximide (i.e. Rovral), anilinopyrimidine (i.e.Vanguard/Scala), and strobilurins (i.e.Abound). It is important to note that each class of fungicides should not be used in succession as rotating will help reduce the chance of resistance. An example of a two spray program could be a 5-20% bloom with Rovral+ Oil and a 100% or petal fall spray with Vanguard.
I have received a few calls about the use of Pristine for brown rot control. Pristine provides excellent brown rot control due to the fact that it contains two different fungicides, strobilurin and boscalid. These compounds were mixed to help reduce the chance of resistance. Pristine is excellent – as well as and other strobilurins (i.e. Flint/Abound/Gem) – in controlling other foliar diseases such as scab, shot-hole, anthracnose, and rust. Since strobilurins can be used to control these diseases AND the optimal timing for scab, rust, anthracnose, and shot-hole tend to be post petal fall, I would advise saving the strobilurin class (this includes Pristine) for the 2-5 week post petal fall application. Back to back applications of strobilurins should be avoided.
Please note that this is not an endorsement for any of the trade names listed, nor does the ommision of of specific trade names reflect the view of the author. Refer to your local chemical dealer or manufacturer for specific fungicide products available.