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MORE ABOUT NEMATODES AND FRUIT TREES 



R.A. Rohde 

 Department of Plant Pathology 



A university student majoring in pomology probably wonders 

 sometimes how, with all of the potential problems, a new orchard 

 is ever established. Problems with soil structure and fertility, 

 drainage, toxic decomposition products from fruit tree roots, 

 soil fungi, bacteria, viruses and nematodes can all injure young 

 trees. Sometimes the injury has a name such as crown gall, 

 collar rot or SARD (specific apple replant disease) but more often 

 the result is slov; or uneven growth that is difficult to diagnose, 

 or even measure. Sometimes trees die from winter injury but were 

 weakened by poor growth the previous summer. 



Nematodes are one of the many factors contributing to the re- 

 plant problem. Nematodes are microscopic worms which live in 

 the soil along with bacteria and fungi and feed on root tips. 

 The feeding process injures or kills root tips and leads to pro- 

 blems of water and nutrient absorption. The resulting wounds 

 usually become infected by root rotting fungi. In addition, 

 some nematodes can transmit virus diseases. 



A vigorously growing, mature tree can support a large number 

 of nematodes without showing any symptoms. However, trees coming 

 from the nursery, especially those in poor condition or being 

 planted under adverse conditions, cannot tolerate this damage. 

 Experiments at Cornell University and elsewhere have shown that 

 the head start given to small trees by soil treatment is never 

 lost even when high nematode populations return after a year or 

 two . 



Soil samples from Massachusetts orchards always contain plant- 

 parasitic nematodes, usually of several different species. The 

 three most common, and most injurious, are the lesion, dagger 

 and ring nematodes. 



Lesion nematodes, Pratylenchus spp., migrate through the inner 

 root tissues breaking them down as they feed. Injury on peach 

 trees is much more severe than on apple because peach roots contain 

 the cyanide-producing compound amygdalin (also known as laetrile) . 

 The cyanide produced in injured tissue increases the amount of 

 damage. Tissues killed by lesion nematodes are quickly invaded by 

 root-rotting fungi and bacteria. 



Dagger nematodes, Xiphinema americanum , have spears which pene- 

 trate into the root tip and cause it to swell and stop growing. 

 Dagger nematodes can transmit the virus that causes peach stem pit- 

 ting or apple brown line and theoretically only one infective nema- 

 tode is necessary. The virus is not common in Massachusetts, but 

 it is present. 



