peach trees which had large crops in 1978 were injured more severely 

 than younger trees commencing to bear. 



Paraquat had been applied annually to control grass and broad- 

 leaf weeds under all peach trees at the HRC but they also had re- 

 ceived periodic applications o£ hay mulch, which was last applied 

 in 1977. Thus, mulch did not prevent damage. 



Frequently roots of fruit trees appear more susceptible to 

 winter injury in dry than in wet soils. However, at the HRC, the 

 injury to apple trees was worse on heavy, poorly-drained soil. 

 Thus, the combination of poor soil aeration, 8 inches of rain in 

 January and no snow cover intensified the problem of root injury. 

 Nevertheless, trees were injured on well-drained soils both at the 

 HRC and commercial orchards or where ledge prevented deep rooting. 



It was not possible to determine whether rootstocks differed 

 in susceptibility. An interplanted block of mature trees on seed- 

 ling roots and M.7 was equally damaged in a commercial orchard. In 

 other blocks in the same orchard, "filler" trees on MM106 died where- 

 as the injury to those on M.7, MMlll, or M.2 rated from none to 

 medium. At the HRC soil rather than rootstocks appeared to be the 

 more important factor contributing to winter injury. No Miller- 

 spur or Empire trees with an 8-inch interstem of M.9 on Antonovka, 

 Mlvllll or Ottawa 11 planted in 1976 on well-drained soil were injured 

 even though soil temperature at 8-inch soil depth went to 19° F. 

 However, Mcintosh and Delicious on MM106, M.7 or M.26 planted the 

 same year in the same block, were severely injured where the soil 

 is poorly drained due to a hardpan underneath. In other blocks on 

 MM. 106, M.7 or M.26, the injury to roots was clearly associated with 

 areas having poorly drained soil. 



Summary 



Growers have become concerned because of the winter injury to 

 roots, especially in the absence of sod under their trees because of 

 annual use of a contact herbicide such as paraquat plus a soil steri- 

 lant (diuron or simazine) . Studies have shown that soil temperatures 

 in winter can be higher under a sod or sod-plus-mulch than under bare 

 sod. However our peach trees at the HRC were severely injured last 

 winter in spite of a heavy residue of mulch. We are more concerned 

 about the occurrence of soil erosion and tree heaving on bare soil, 

 which is much more common, than possible v\rinter injury to roots. 



This last summer, we tagged individual limbs and whole trees 

 at the HRC after rating the severity of winter injury. This should 

 enable us to determine the degree of tree recovery in the orchard. 



Hopefully, the combination of excessive rainfall in January 

 and bare soil in early February during a period of sub-zero air 

 temperatures, will not reoccur for many years. 



