- 15 



APPLE REPLANT PROBLEMS 



Joseph F. Costante 

 Plant and Soil Sciences Department 

 University of Vermont 



Difficulties in establishing apple trees on old orchard sites could 

 pose a risi< to tree growth and seriously affect crop performance. The 

 living orchard soil contains billions of soil microorganisms in every hand- 

 ful. Some of these rhizosphere organisms can cause disease problems to 

 roots, especially within the top 1^ inches of soil. Seventy to eighty per- 

 cent of a soil's metabolic activity is generated by organsims like nemato- 

 des, bacteria, fungi, and act i nomycetes. Apple replant disease (ARD) refers 

 to poor growth of apple trees on old orchard soils caused, in large part, by 

 harmful rhizosphere organisms. ARD is a major problem of apples in New 

 England and will likely increase as orchard sites are replanted over and 

 over. The level of severity depends on such factors as orchard age, pre- 

 vious host crops, soil type, and extent of damage on old tree roots. 



Any losses incurred in tree replacement due to replant failure presents 

 a serious obstacle to growers. In view of the escalating cost of fruit pro- 

 duction, and the limited supply of suitable sites for orcharding in New 

 England, these replant difficulties and their control are being given 

 increased attention. 



Symptoms of Replant Injury 



Numerous factors, other than soil microorganisms, contribute to diff- 

 culties in replanting apple trees. These include poor, injured and diseased 

 planting stock, improper planting, irregular cultural practices, unbalanced 

 soil nutrition, poor drainage, impaired soil structure, and allelopathic 

 effects. 



The above-ground symptoms of injury from root-feeding nematodes and 

 rhizosphere organisms are generally those which result from partial destruc- 

 tion of the root system. These include stunting, foliage wilting and 

 yellowing, and, in extreme cases, death of the tree. Some parasitic nema- 

 todes and rhizosphere organisms may cause swollen and distorted roots while 

 others cause dark colored lesions, killing large numbers of unsuberized 

 feeder roots. Trees may remain severely stunted for years or during their 

 entire life. Although the majority of apple trees usually experience 

 improved growth with age, they rarely become as productive as healthy trees 

 on sites with low populations of parasitic nematodes. 



It appears that there are a number of different replant problems and 

 that many organisms are involved. Causal organisms will vary among each 

 orchard's habitat and even between trees. Many failures in replanting trees 

 have been attributed to drought, cold, or winter injury. Usually trees suf- 

 fering from ARD are much more susceptible to drought and cold injury than 

 are similar trees with uninjured roots. 



