Apr 7, 2022Scab and fungicide spray schedule options in pecan
Growers get antsy this time of year and are itching to spray. However, unless you are in a very scabby location with highly susceptible cultivars, there’s no reason to begin spraying at this point. We’re likely at least 10 days to two weeks away from needing to begin fungicide sprays in most areas.
Yes, leaf scab can be important, but it’s not as important as nut scab, and if you are going to save on your fungicide bill, the time to do that will be early in the season. The earlier you begin, the more sprays you will have to make. Once June arrives, there will be little margin for error, and stretching out sprays during nut sizing becomes very risky on susceptible cultivars from June onward.
Disease development requires the host and pathogen to be present and the conditions to be suitable for the pathogen to grow. If you want to assume the pathogen is present, consider that our host – the green tissue of pecan trees in the form of foliage – is not fully present yet.
So, what would you really be spraying at this point? Now, let’s look at the conditions required. Scab grows within a range of 59-95° F (optimal temp. is 59-77° F) in the presence of free moisture (usually for at least 12 hrs). Since April 1, the temp ranges have been 53-75° F; 49-68° F; 48-75° F; 49-80° F; and 57-77° F with zero rainfall. So our temperatures are getting close, but haven’t stayed within range for very long, and the free moisture has been lacking.
There is significant rainfall forecast over the next few days but again, there’s not much leaf growth out there yet. Following the rain, temps are forecast to fall to 44-67° F through the weekend and then warming by the first of next week.
But even if it warms up, disease won’t develop without the moisture, and it will be yet another week before there is enough growth out to worry about. So, you still have some time before you need to spray as I write this on April 5.
To be perfectly clear, the schedules offered here are not an advocation for a strict calendar spray schedule. Obviously, scab development is based on the period of leaf wetness, which is not based on a calendar date, but is influenced to a high degree by the frequency of rainfall. These schedules are simply to be used as a framework on which to base your program. They incorporate what we know about the best use of each fungicide.
Some fungicides, like Phosphite and the group 3 + group 11 materials, have better activity on leaf scab. Others, like Elast, Tin, and Miravis Top offer the highest degree of nut scab protection. There are also other labeled materials that could be worked into the schedule. The following is simply an example.
If it rains frequently, you need to tighten up your schedule on medium and high susceptibility cultivars. If it is relatively dry and the pressure is low, you can space the schedule out more early on in the season, especially with medium susceptibility cultivars. Once nut sizing begins (June), susceptible cultivars should never go more than 14 days between sprays, even if conditions are dry, because we can get enough humidity and wetness from the dew to drive scab development even without rain. With frequent rainfall, you will need to tighten up to 10 days or less between sprays. A good rule of thumb is to tighten the spray interval when you get two or more rain events (0.10″ or more) before the 14-day standard interval is up during nut sizing.
As we did last year, we’ve broken the cultivars down into three main categories (low, medium and high) as seen below. The fourth category – medium/high – consists of cultivars that could fall into either of these two categories.
Under most conditions, they would have no scab problems under a regular spray program, but in certain locations (below Highway 280, at low elevation, in crowded orchards), they will scab more and would need a high input program. Growers should use their own judgement and experience about where to place these. I am available to discuss this with any growers who are unsure about where their orchard fits.
Low input cultivars are those with a very high degree of scab resistance – think Elliot, Excel, Lakota. These require a bare minimum of sprays – three applications at most – primarily to help manage minor diseases aside from scab like powdery mildew, anthracnose, downy spot, etc., and to assist in maintaining scab resistance
Spray 1: Phosphite ~ mid-late April
Spray 2: Phosphite ~ mid-to-late May
Spray 3: 11 + 3 mix ~ early-mid June
These are cultivars that will require fungicide sprays to manage the disease, but on which scab is usually easily managed without an intensive spray program in most locations. Some of these cultivars can fall into the high susceptibility category in locations with a history of scab on these particular cultivars, or under situations of low elevation, poor air flow or frequent rainfall. Use your best judgement with regard to where these cultivars fit for your own location.
Scab on these cultivars should be controlled with 7-8 sprays. If excessive rainfall is occurring throughout the nut sizing period, you can shorten your interval and extend the program out further by continuing to rotate Miravis Top and Elast/Tin in the example below.
Spray 1: Phosphite ~ mid-late April
Spray 2: Phosphite OR 11+3 ~ mid May
Spray 3: Miravis Top ~ early-mid June
Spray 4: Elast+Tin OR phosphite ~ mid-late June
Spray 5: Miravis Top ~ early-mid July
Spray 6: Tin OR Elast+Tin ~ mid-late July
Spray 7: Miravis Top ~ early-mid August
These are cultivars that we know must be sprayed intensively in order to produce the crop. They will require at least 10 fungicide sprays and likely more in many locations. If you need to extend beyond spray 10, continue rotating with Elast/Tin, but consider substituting a group 3 + group 11 for Miravis Top since no more than four sprays with Miravis Top are advised in a given year. Miravis Prime may also be an option for rotation with Elast/Tin if available. Bear in mind that some cultivars listed in the moderate category may fall into the high category in some locations.
Spray 1: phosphite
Spray 2: phosphite
Spray 3: 11 + 3 mix
Spray 4: Miravis Top + phosphite
Spray 5: Elast + Tin
Spray 6: Miravis Top
Spray 7: Elast + Tin
Spray 8: Miravis Top
Spray 9: Elast + Tin
Spray 10: Miravis Top
As mentioned previously, there are certainly more fungicides labeled for pecans than what you see listed in the examples above. Their exclusion from these examples does not mean they do not control scab. To the contrary, many are very good fungicides and could be rotated into a program just as easily as what you see above.
Based on Dr. Tim Brenneman’s data, we feel that these chemistries applied at the stage the crop will be in during the times shown above will offer maximum protection from leaf and nut scab. Group 3 and Group 11 fungicides are those containing both a triazole and a strobilurin fungicide chemistry (think Absolute, Stratego, Quadris Top, Amistar Top, Quilt, Brixen, Custodia, TopGuard EQ, and others).
– Lenny Wells, University of Georgia
Photo: University of Georgia