Rich Marini, Penn State Plant Science
We have known for more than 80 years that post-bloom temperatures can influence harvest date, but not until recently have we realized that post-bloom temperatures can also affect fruit size at harvest.
What might we expect in 2012?
Peach fruit size at harvest depends on the number of cells and the size of those cells within the fruit. Commercial peach growers use various cultural practices to alter both factors. Small-fruited varieties usually have fewer cells than large-fruited varieties, so varieties with large fruit within a harvest season (early-, mid- and late-season) can be selected. Early-season fruit growth is primarily due to cell division which is enhanced by removing excessive fruiting shoots while pruning along with bloom thinning and post-bloom fruit thinning. Cell size can be increased by minimizing water stress during the final swell because late-season fruit growth is due primarily to water accumulation within the cells.
In 1930, Professor Blake published a New Jersey experiment station bulletin where he summarized 10 years of bloom and fruit development observations for ‘Elberta’ trees growing in Vineland, NJ. He reported that between 1910 and 1919, the date of full bloom varied from March 29 to April 28, but for nine of those years the harvest date varied only from August 20 to August 29. So during this period, bloom date varied 29 days but days from bloom to harvest varied only 21 days. Days from bloom to harvest varied from 144 days to 123 days because days from bloom to harvest tended to increase as bloom date was advanced. The year 1917 was unusual because trees bloomed on April 23, but were harvested on September 6, so there were 136 days from bloom to harvest. Professor Blake noted that 1917 was an unusual season because May was abnormally cold. Blake’s explanation was that fruits develop slower when bloom is early because post-bloom temperatures tend to be lower for seasons with early bloom. He also observed that harvest tended to be delayed by about 5 days when trees received high rates of nitrogen fertilizer. In another study the fruit development period was recorded for 8 peach varieties over a 10-year period. Days from bloom to harvest varied only 8 days for the variety ‘Alton’ and it varied 25 days for ‘Reeves’. This indicates that some varieties are able to grow at nearly the normal rate even when post-bloom temperatures are relatively low.
Since 1930 there was little research on the effect of temperature on peach fruit development. California experienced early peach harvest in 2004, when record high temperatures were recorded during full bloom. During that season, many growers also reported that fruit size was disappointing. This unusual year prompted Dr. Ted DeJong, at the University of California at Davis, to study the importance of spring temperatures on peach fruit growth and harvest date.
Since there is little foliage on a peach tree to produce carbohydrates during bloom, early-season peach fruit growth primarily depends on stored carbohydrates within the tree. During late summer sugars resulting from photosynthesis are transported to tree roots and woody tissues where they are stored as starch for the winter. During early spring, starch is converted back to sugars which are used for early-season growth of all tree parts, including fruit. Spring starch conversion is usually slow due to cool root temperatures, so there is likely a shortage of carbon available for fruit growth. Respiration rates, that require carbohydrate, increase with increasing air temperatures. Since high heat unit accumulation during early spring increases the rate of fruit development and respiration without a corresponding increase in carbohydrate supply to support fruit growth, the California researchers hypothesized that early-season fruit growth would be negatively affected by high spring temperatures. They used 20 years of climate and fruit growth data from three locations in California to test this hypothesis.
They found that full bloom date (FBD) and the reference date (RD) for a given location varied by about 3 weeks during the 20-year period. Reference date is the date on which 80% of sliced fruit have hardened pits near their distal end, plus 10 days. The average time from FBD to RD varied only from 69 to 72 days depending on location. There was a negative correlation between cumulative growing degree hours for the first 30 days after bloom (GDH30) and the days from FBD to RD. For each 1,000 GDH30 the number of days from FBD to RD decreased by 4.9. Fruit size at the RD was also negatively related to GDH30; for every increase of 1,000 GDH, fruit diameter declined 1.0mm. Therefore growers may substantially improve fruit size by early thinning in years with higher spring temperatures and heavy fruit set.
In another study they used GDH30 to develop a model to predict harvest dates. They found that the number of days between full bloom and harvest decreased with increasing GDH30. This supports the results reported by Blake about 80 years earlier. They found that for every 1,000 GDH30, the number of days from full bloom to harvest decreased by 3.5 days for peaches and 4.1 days for nectarines.
Results from a cooperative study involving six locations (CA, SC, GA, MD, NY, and NJ) over three seasons, using ‘Cresthaven,’ also indicated that large fruit size was associated with cooler temperatures (Johnson, et al. 2011). They found for each additional day from bloom to harvest, average fruit weight increased 7.6 grams or 0.268 ounces. The longer the fruit are on the tree the larger the fruit at harvest. Since days from bloom to harvest are effected by temperature, we can combine results from several studies to get an idea of the effect of temperature on fruit size at harvest. If we record 1,143 growing degree hours less than normal during the month after bloom, we can expect that harvest will begin 4 days later than normal and the average fruit will weigh 1.07 ounces more than normal.
So, what might we expect in 2012? The spring weather has been so unusual that all this research may not be relevant for 2012. For most of Pennsylvania, peaches began blooming about 4 weeks earlier than normal and then we experienced below normal temperatures. Since temperatures during the 30 days after bloom are most important for fruit development, we might expect that for most varieties the number of days from bloom to harvest may be longer than normal, so harvest may begin only about 10 to 14 days earlier than normal. A complicating factor is that there probably has been frost damage in many orchards. This early “thinning” may advance harvest because fruit on lightly cropping peach trees typically ripen 7 to 10 days earlier than fruit on trees with heavy crops. So harvest may begin earlier than expected despite the cool post-bloom temperatures. The good news is that I would expect that the combination of cool post-bloom temperatures plus some flower bud damage should result in good fruit size this year.