Dust from mowing and tilling can reduce pistachio fruit set

Mar 19, 2020

Bloom is an important time in all crops, and pistachio is no exception. Some of the activities that are often performed around bloom is mowing or tilling row middles, as well as application of burndown herbicides. However, recently released results from a research project led by Lu Zhang (and also included Bob Beede, Gary Banuelos, Christopher Wallis, and Louise Ferguson), formerly a postdoctoral scholar at UC Davis and now a professor at Oklahoma State, may make you want to rethink the timing of those activities. While I'm writing about this, I was not involved in the research.

The researchers, after observing dusty bloom conditions during the long term drought that ended in 2017, sought to determine if dust decreases pollination, fruit set, blanking, or split percentage. In order to control conditions completely, they opted to hand pollinate pistachio flowers with pre-determined pollen and dust mixtures. This is important to note, as they were not actually measuring pollination in ‘real world' dusty conditions. They used 100% pollen, 100% dust, and mixtures containing 50%, 75%, and 94% dust. Because herbicides can sometimes be applied around this time, they also pre-treated some soil with the postemergent herbicides glyphosate and saflufenacil, then mixed the treated soil with pollen (in a 50-50 mixture). In order to understand what it looks like when flowers are prevented from being pollinated, they also bagged some pistachio flower clusters.

Before getting into the results, it may be helpful to have a refresher on how pollination and fruit set works in pistachios.  Pollen is released by male pistachio trees, which travels via air to land on a female flower. The part of the female flower that catches the pollen, called the stigma, is sticky, which helps pollen land on it, and more than one pollen grain is needed for successful pollination. Once the pollen lands, it germinates, much like a seed. It produces a hollow structure called a pollen tube, which grows through the female flower to reach the ovary. A sperm cell travels through the pollen tube to pollinate the egg cells, and the resulting embryo develops into a seed. Multiple pollen grains are needed, as not every grain is able to successfully grow to reach the ovary.

After pollination occurs, the fruit begins to grow: the hull and shell fully expand, then the embryo begins to fill it. Far more fruit sets at the beginning of the season than what makes it to the end. Typically this is because the tree sets far more fruit than it can support, and the tree will abort fruit later in the spring to balance its load. A pistachio flower cluster will have 100 to 200 individual flowers, but on average, only 14 fruit make it through to harvest. Early season insect damage can also result in fruit drop, though depending on when this occurs, it may not cause economic damage (as the damaged fruit drops off, and fewer fruit are dropped during the rebalancing period). 

Pistachios are unique in that the fruit and shell will grow before the embryo begins to grow, which starts in June. This leads to unique issues with blanking and splitting percentage. If a pollination event begins, but is unable to successfully be completed, the hull and shell can be stimulated to grow without the following embryo growth (called parthenocarpy), which is one cause of blanking. Another is when the embryo begins growth but aborts later on, which is typically due to stressors like drought. While split percentages are not fully understood, it is thought that insufficient heat hours can contribute to this, as well as insufficient carbohydrates.  The evidence for split percentage is circumstantial and is based on the fact that split percentage is higher in OFF years than ON.    

The researchers were interested in looking at blanking to see if dust could trigger parthenocarpy, and split percentage to see if dust interfered with successful embryo growth. The results were discouraging. Pollen contaminated with dust had less of an ability to germinate (lower viability), which reduces the chance that pollination will successfully occur. In addition, dust physically blocked the pollen from reaching the female stigma, which sometimes but not always reduced the fruit set. Where the effects really became clear was when looking at the June drop and blanking rate: as the amount of dust in the mixtures increased, the June drop rate increased and blanking (generally) increased. Split percentage also decreased when the dust percentage was 75% or higher, which may be due to decreased pollen viability. If the pollen was weaker, it could have resulted in weaker embryo growth. Flowers that encountered herbicide contaminated dust did not have any fruit that continued to harvest, which the researchers found through electron microscopes was because the stigma became deformed after the application of herbicide contaminated dust. 

% Fruit set% nut dropBlank %% Splits
Control (bagged) 3.6 46.4 83.3 0
Pollen 18.2 11.5 10.9 51.7
50% dust mix 17 10.4 16.5 61.7
75% dust 24.2 15.4 42.6 30.9
94% dust 11.5 17 39.3 31.6
Dust 3.3 0 80 10
50% herbicide contaminated dust 5.2 100 N/A N/A
Adapted from Zheng et al. (2019).  There was no data for blank and split nuts for the herbicide contaminated dust treatment, as no fruit made it to harvest.

This study also provides very good evidence that dust may contribute to blanking and lower split percentages.  Remember that is not the sole cause of blanking, however: pollen that dies before pollen tube growth is completed and drought stress can cause it as well.

There is yet another caveat: while the results are clear that brushing dust onto pistachio flowers can increase the blank percentage, it does not show whether tillage or mowing actually generate enough dust to interfere with pollination. To put it another way, we just don't know whether this is happening in the real world. From what I've personally observed, tilling bone-dry ground creates a heck of a lot of dust that could travel high enough to get into the tree canopy.  However, given the strength of the results and the short time period when pollination occurs, it would be a good idea to avoid mowing or tilling during bloom, especially if the orchard has had herbicides applied recently. While working wet soil reduces dust, it can cause compaction and smearing, and I do not recommend working the ground in that condition: it's better to just work around bloom. It is also an excellent idea to avoid applying herbicides during bloom, especially since nearby bee colonies can sometimes collect pollen from pistachio orchards. 

If you are interested in reading the actual article, it is available to the public at: https://journals.ashs.org/hortsci/view/journals/hortsci/54/11/article-p1967.xml

Zhang, L., Beede, R.H., Banuelos, G., Wallis, C.M. and Ferguson, L., 2019. Dust Interferes with Pollen–Stigma Interaction and Fruit Set in Pistachio Pistacia vera cv. Kerman. HortScience54(11), pp.1967-1971.

By Phoebe Gordon
Author - Orchard Crops Farm Advisor