Disease Update: Fire Blight, Apple Scab, and Bitter Rot Management are Necessary

Vigilance is needed, and protection is most optimal when applied before rain events. The following update includes a healthy refresher of what to know and keep in mind for disease management during the next few weeks. Especially management strategies for those critical diseases that can keep folks up at night—fire blight, apple scab, and bitter rot.

Minimum requirements for fire blight blossom infection

For blossom infection to occur, the following criteria must be met in the following order:

1. Flowers must be open with petals intact (flowers in petal fall are resistant)
2. An accumulation of at least 198 degree hours above 65ºF
3. A wetting event as dew or rain
4. An average daily temperature of 60ºF

Streptomycin

How does streptomycin protect open flowers from fire blight disease?

Insects are attracted to the ooze from active overwintering fire blight cankers. They unwittingly pick up the bacteria on their bodies and move it to flowers as they are busy pollinating. The bacteria on the insect bodies are deposited on the flower stigmas. Here, the bacteria replicate and reach high numbers. A wetting event, such as dew or rain, will then wash the bacteria from the stigmas into the open nectaries. When streptomycin is applied to open blossoms, it kills the bacteria on the stigmas.

Streptomycin has partial systemic properties, such that any bacteria that may have moved into the nectaries will also be killed. In addition, there is about a 48-hour window of activity for streptomycin. Applying streptomycin resets the clock, you're cleaning up the flower. If you apply streptomycin today and it rains tomorrow, there is no need to worry about streptomycin being washed off; you had already killed the bacteria in the flower when you made your streptomycin spray. Streptomycin has done its job.

Therefore, it is critical to apply streptomycin Before the wetting event. It's a lot easier to kill the bacteria before it has moved into the plant via the nectaries. Don't apply what you know about fungicides and controlling fungal diseases with controlling fire blight. You want fungicides to persist through and after a rain event because that is when control is most critical; this isn't the case when applying streptomycin controlling for fire blight. In short, streptomycin packs its biggest punch at the moment and shortly after application.

Plant Defense Elicitors

How does applying plant defense elicitors, such as Actigard, help with fire blight management?

When you tank mix streptomycin and Actigard and apply the mix to your open blossoms, Actigard will be absorbed by the plant tissues. After it's absorbed, the plant defenses will be turned on in approximately 48 hours. This signal can persist for a period, depending on the size and age of the tree. Actigard enhances the efficacy of your blossom blight protection program. The streptomycin will kill the bacteria in the flower, and the activated plant defenses will further protect the tree from infection from any wayward bacteria that escape the streptomycin application.

Actigard is not a substitute for streptomycin—Actigard does not kill the bacteria. Depending on the season, streptomycin may not be 100% effective. We observe this in our research trials. Actigard can be viewed as extra insurance to help you get through the bloom season, especially if there is rattail bloom or a protracted bloom. If bloom hangs around a while, you want to be sure to apply streptomycin if five to seven days have elapsed since the last streptomycin spray. Remember: streptomycin kills the bacteria. Regalia is another option as a plant defense elicitor; however, please note that Regalia at 2 qt/A will cause damage to the flower petals when applied during bloom; however, this does not affect fruit finish.

Fire Blight

How long do I need to worry about fire blight this season?

Growers need their fire blight management vigilance on high alert from bloom through approximately early June. The period post bloom is very critical for the establishment of fire blight. As a result, a proactive shoot blight program is warranted. This can be through the application of repeated Cueva (2 qt/A) sprays or Apogee/Kudos (2 oz – 12 oz/A). The rates for Apogee/Kudos will depend on the age and size of your trees. We have preliminary data showing a tank mix of Apogee/Kudos (2 oz/A) with Regalia (2 qt/A) reduced shoot blight significantly compared to Apogee/Kudos (2 oz/A) or Regalia (2 qt/A) by itself.

We applied this combination three times, every ten days starting at petal fall. When using Apogee/Kudos, growers will need to monitor growth to determine the number of extra applications that are needed. It takes 10-14 days for the effect of Apogee/Kudos to kick in. Depending on when you begin your treatments, monitor your trees closely. If disease conditions are favorable (especially when very warm, wet temperatures occur), a Cueva application may be necessary to keep trees protected before the Apogee/Kudos takes effect.

Apple Scab

What are the best options for managing apple scab from bloom through petal fall?

We are in a period where disease pressure will be peaking if warm, wet conditions occur. We are recording exceptionally high mature scab spores being available, and this will continue for about the next week or two. If rain is in the forecast, growers are encouraged to use the FRAC Group 7 fungicides tank mixed with a rainfast mancozeb for the next 2-3 weeks. Complete sprays are recommended; however, if half sprays are used, shorten the interval in between sprays. If disease conditions persist, no more than 3.5 days in between half sprays should occur. FRAC Group 7 fungicides include Aprovia, Fontelis, Sercadis, Luna Sensation, Luna Tranquility, Merivon, and Pristine. Use fungicides in FRAC group 3 and 9 (e.g., Inspire Super, Indar) as rotation partners during this period.

Bitter Rot

When should bitter rot management begin?

Bitter rot was a problem last season. Our research to date has shown bitter rot spores being available as early as bloom. Consequently, a good fungicide program during bloom will be necessary, especially if conditions are frequently warm and wet. We have also observed the number of leaf wetness hours is also very important for disease establishment. If bitter rot was a problem last season, the best fungicide to use during bloom through petal fall is Merivon (two – three applications, depending on weather conditions). Omega (6.9 fl oz/A) is also another option; this can be rotated with Merivon during this early season period. Tank mix with a rainfast mancozeb.

For commercial fruit growers, please note: When controlling for disease, weather and tree growth conditions need to be monitored at a local level within one's own orchard. Before chemical products are applied, be sure to comply by obtaining the current usage regulations and examining the product label. Product information can be easily obtained from CDMS.