The Good and Bad of Abundant Precipitation
Pecan tree with tumor disease. (Photo by Charlie Graham)
We are always concerned with adequate water during the nut-sizing phase in mid-summer. Pecan varieties genetically differ in their final size, but several factors including tree vigor, soil fertility, crop size and water availability also influence nut size. The LSU Pecan Research Station has received over 60 inches of precipitation through the first nine months of 2016 and nuts appear to be well sized this year.
The water stage in late July and early August is one of the most common times for environmental stress to increase nut drop. Any stress received by the tree as the nuts move from size development into kernel formation can result in major fruit drop. While we are usually concerned about inadequate moisture at this time of year, excessive water resulting in water-logged soils or rapid water uptake at shell hardening can affect nut abortion. We have seen a major problem with water-split of ‘Oconee’ nuts recently. I have received reports from both irrigated and non-irrigated orchards in Louisiana. At the LSU Pecan Research Station, many ‘Oconee’ trees aborted more than 50 percent of their crop as the trees received heavy rains at just the wrong time. No other variety on the station appeared to be adversely impacted by the rains in late August.
Fruit development in pecan is a process that requires high energy, which can induce a considerable amount of stress on older trees. Excessive stress can result in incomplete kernel development that has been associated with shuck deterioration. The first symptom of shuck decline is a thin, dark, necrotic line on the inner shuck, which then spreads to the outer surface of the shuck. The shuck looks water-soaked in appearance before turning black. Affected pecans will often stay in the cluster, but will often have premature shuck opening. While the condition is often associated with excessive fruiting, it can also occur in trees with a moderate crop when coupled with inadequate soil moisture. But that doesn’t appear to be a problem this year as good soil moisture has shuck split progressing nicely.
Several breeding lines and early varieties such as ‘Pawnee’ have already passed 50 percent shuck split and I have observed very little shuck decline and no vivipary so far this season. Several times over the last decade we have seen ‘Pawnee’ have delayed shuck split due to low moisture availability in August and September. This delay was generally associated with an increase in vivipary or sprouting of the nuts on the tree, sometimes resulting in a loss of one-quarter to one-half of the crop.
A common floor-management strategy for pecan orchards is to use a grass alley with a vegetation-free strip in the tree row out to the dripline of the trees. Grass is beneficial in an orchard because it reduces erosion in sloped orchards, provides a good environment for proliferation of beneficial insects and, during periods of excessive rainfall, it helps provide a more traversable surface for spraying and mowing equipment. Warm temperatures and above-average rainfall have resulted in the rapid and excessive growth of grass in orchards. Grass height can be controlled by mechanical mowing, livestock grazing, hay production or chemical mowing.
Several different herbicides can be used to establish and maintain the vegetation-free strip in a pecan orchard. Now is a good time to evaluate how well your herbicide program performed this year. Growers relying strictly on glyphosate probably found themselves having to spray an extra time or two this year. For those of you that spent the extra money to apply pre-emergent herbicides, how well did they keep the grass under control? Growers relying solely on mechanical mowing to control grass this year are probably still trying to get caught up on their grass control. Excessive rainfall, in amount and duration, made it difficult to get equipment into the orchard and keep the grass mowed to a proper height. We are still trying to get everything mowed before shuck split advances to the point we can no longer mow because we will be destroying nuts on the orchard floor.
Of course, foliage retention is extremely important for storing reserves for next year’s crop. Black aphid and scorch mite pressure was not as severe this growing season as last year. And while the numerous rains did delay insecticide and miticide sprays, most growers did a good job of getting their equipment into the orchards and controlling populations before they got out of hand. The numerous rain showers we received minimized the amount of honeydew on the leaves and sooty mold has not been a major problem.
There has been some foliage loss due to Gnomonia leaf spot, as well as some leaf loss in ‘Pawnee’ and ‘Elliott’ trees due to Neofusicoccum. The $64,000 dollar question has been how much fertilizer was lost due to the rain this year and should I make another nitrogen application? Growers with large crops have been applying fall nitrogen applications to make sure there is adequate nitrogen in the tree for next year’s crop.
This summer I have received an above-average number in calls asking me to evaluate dying trees in orchards across the state. Many of them have been suffering from tumor disease, but I have been unable to confirm if this increase is associated with the excessive rain we have received this year. Tumor disease causes distinct outgrowth on the trunk and major limbs of a pecan tree. This altered growth does not appear to affect the infected tree’s production, but definitely affects the appearance. Research on the causal agent is almost nonexistent, as it has never been reported to have a major economic impact in an orchard. Phomopsis has been suggested, but not confirmed. Typically, I have only observed it in trees over 50 years old, but several of the affected trees this year were planted in the 1970s and 1980s.
Overall, the above average rain has positively affected the pecan crop by increasing nut size, reducing sooty mold growth, and allowing shuck split to progress on a normal schedule, as compared to the delays we have seen over the past few years. However, growers not applying adequate fungicide sprays have been negatively impacted by pecan scab.
