16 - 



Agents Causing Destruction of Feeder Roots 



Researcti has shown that ARD problems in New England orchards are caused 

 by at least two biological agents. The first organism is the lesion nema- 

 tode, Praty lenchus penetrans , and the second is on incompletely identified 

 root rhizosphere organi sm(s) . Although this causal organism(s) has not been 

 completely identified, findings indicate that root-pathogenic fungi will 

 destroy unsuberized roots and cause replant problems. Fusar ium spp., 

 Pythium spp., Phyt ophthora spp., Thielaviops is spp., and Rhizoctonia solani 

 are five of the fungi implicated. 



Important Parasitic Nematodes 



The microscopic roundworms that live in soil or water are l<nown as 

 nematodes. Most are free-living and pose no disease problems, whereas 

 others are parasites of apple roots. Some species of nematodes inadver- 

 tently introduce pathogenic root invading microorganisms into the plants 

 while feeding. in other instances, the nematodes themselves cause the 

 disease, disrupting the flow of water and nutrients in the vascular system. 

 This can result in root-knot or deprivation of the above-ground parts, ulti- 

 mately causing stunting. 



Root-Lesion Nematodes ( Praty lench us spp.) Lesion nematodes are con- 

 sidered the most damaging nematode pathogens and are probably present in all 

 New England apple orchards. They are endoparas i tes which live and feed 

 inside feeder root tissues, migrating through soil to go from root to root. 

 P. penetrans is the most economically important species causing critical 

 damage to roots of newly planted trees. Recent data from Vermont show that 

 the les ion-nematode affects all major rootstocks (M.9, M.26, M.7, MM106, 

 MM111 and seedling) growing in light and heavy soils. Generally, most 

 orchard tree roots have a low number of P_^ penetrans nematodes and cause 

 little or no damage. Injury occurs when population levels drastically 

 increase into the hundreds (per gram of root tissue). Some of the factors 

 affecting nematode populations are previous crop, soil type and condition, 

 health of host, winter climate, and unexplained variation. 



Dagger Nematodes ( Xiph i nema spp.) The dagger nematodes are ectopara- 

 sites which feed from the root surface. The most commonly found orchard 

 species is X. amer icanum which causes reduced growth and necrosis of feeder 

 roots. This nematode transmits tomato ringspot virus resulting in apple 

 union necrosis. 



Less important ectoparasite nematodes which have been reported to 

 contribute to ARD problems in New England include Pin nematodes 

 ( Paraty lanchus spp.). Lance nematodes ( Hoplolaimus spp.), and Ring nematodes 

 ( Cr iconemoides spp.). 



Assessment of Replant Problems 



Certain procedures should be followed prior to replanting an orchard 



site to gain insight into soil-based problems and to correct them as quickly 



as possible. The key to success is a program of sound horticultural prac- 

 tices. 



