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Practical Grower Lynn Long on .... |
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Practical Grower Lynn Long on .... |
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Replacing an unproductive block of cherries with new cherry trees or interplanting an established block may not always lead to the high yields growers hope for. Too often, a malady referred to as "replant disease" can make a disaster out of an already expensive investment.
In replant situations where cherries follow cherries, it is not uncommon for new trees to be stunted. Symptoms of "replant disease" include slow growth, low vigor, and low yields. In most cases, trees will struggle with these symptoms their entire life, leaving the grower with an expensive investment and no remedy.
The cause of "replant disease" is not totally understood.
Most likely, a number of factors adversely affect tree growth.
in the Mid-Columbia area of the Pacific Northwest, we believe
one potential cause of these problems is root lesion nematodes,
Pratylenchus penetrans. This potential relationship is currently
being researched by Oregon State University (OSU) scientists.
Soil Fumigation
In a study I conducted between 1989 and 1992, it was shown that
soil fumigation, prior to planting, helps to improve tree growth
and increase early yields (see Figure 1).
Depending on the fumigant, trees growing in treated soil were
53 to 69 percent larger than the control after four years. In
addition, trees growing in soils treated with metam-sodium yielded
143 percent more fruit in the fourth leaf.
Herbicide Strip Factor
Dr. Tim Facteau of the OSU Mid-Columbia Research and Extension
Center in Hood River also found that it is important to avoid
planting into former herbicide strip areas whenever possible.
In a study conducted by Facteau, cherries followed by cherries
grew better when planted in former grass alley-ways rather than
old tree rows when the entire area was fumigated with methyl bromide.
After four years, trees planted in the former grass strips had
trunk cross-sectional areas 7.5 cm² larger, and yielded four
pounds more, than trees planted in the former herbicide strip
(which was also the former tree row).
There are obviously no easy answers to these replant problems.
Avoiding sites where former herbicide strips were located is not
always possible. In addition, methyl bromide is expensive, dangerous,
and targeted to be removed from the market in the year 2000. Metam
sodium, also used in my replant study, is expensive and difficult
to apply due to the large volumes of water needed to incorporate
it into the soil.
Colt Rootstock
Another potential answer to "replant disease" is
to use a resistant rootstock. Last fall, a grower asked me to
look at a side that he had interplanted. The mature trees in the
block were growing on Mazzard rootstock. Interplants were planted
in the spring of 1993 and were, therefore, in their second leaf.
One block was interplanted with Bing cherries on Mazzard rootstock,
with Lapins and Rainier serving as pollenizers. The pollenizers
were growing on Colt rootstock. An adjacent block, interplanted
at the same time, was planted with Bing on Colt and Van on Mazzard.
In all cases, the interplants growing on Mazzard rootstocks were
small and stunted. In fact, they had grown so poorly the first
year that the grower pruned them back hard during the first dormant
season, hoping to stimulate more vigorous growth. The trees did
not respond to this technique and remained small.
Neighboring trees growing on Colt rootstocks, however, regardless
of the variety, were vigorous and strong. In fact, in both blocks,
the cross-sectional area of the trees on Colt was over twice as
large as those on Mazzard. (see Figure 2)
Soil and roots in this block were sampled for root lesion nematodes
to see if they might be a factor in the poor growth and development
of the trees growing on Mazzard. Nematodes were found in the soil
at the rate of 470 per quart and in the Mazzard roots at 192 per
gram of fresh roots. It is suspected that these nematode populations
are affecting tree growth, but this relationship is not yet established.
The Colt rootstock has been around for a number of years but has
never gained wide acceptance in the Pacific Northwest. It is,
however, popular in California and is used extensively in Italy
in replant situations.
At one time, it was thought that Colt would grow a tree somewhat
smaller than Mazzard; however, that has not proven true.
Fruit produced from Bing growing on Colt rootstock was comparable
in size to Bing growing on Mazzard, averaging 7.7 grams compared
to 7.65 grams.
With the current and future limitations associated with the use
of soil fumigants, alternative solutions to this "replant
disease" need to be found. Using Colt rootstocks in replant
and interplant situations may be one potential answer to this
problem.
