THE ROLE OF PYTHIACEOUS FUNGI IN TREE YELLOWING AND DEATH IN TEXAS
EP VAN ARSDEL1 and FH TAINTER2
1Van Arsdel Tree Service, Inc., Tijeras, NM 87059, USA; 2Department of Forest Resources, Clemson University, Clemson, SC 29634, USA
Background and objectives
Urban and woodlot trees, including post oak, water oak, live oak, winged elm and hackberry, growing near Bryan/College Station, TX frequently exhibit abnormal chlorotic leaves followed by rapid death of the tree. Consistent isolations of Pythiaceous fungi (especially Phytophthora species) from these trees indicate that these fungi are contributing to the foliage discoloration, retarded growth, and ultimate death of these trees. These symptoms are distinctly different from those caused by the oak wilt and oak decline fungi. These symptoms are the same symptoms frequently described in publications aboot dying oaks in Texas from 1934 to 1949. This root rot condition is notable in the Post Oak Region where it is associated with oak decline (Cephalosporium), alkali, and salt problems, and where oak wilt (Ceratocystis
Materials and methods
Small roots, and adjacent soil samples, were collected from symptomatic trees and incubated in holes placed in fresh pear (usually Kiefter) or apple (usually Granny Smith) fruits . Fruit tissues developing firm, brown rot were cultured on PCH 121 medium and identity as Pythiaceous fungi confirmed. The local physical environment was studied to detect potentially stressful growth conditions. Direct chemical control with soil injections of metalaxyl and ethazol in the root zones of symptomatic trees was attempled. Additional chemical applications of sulfur, ammonium, magnesium, and zinc sulfates were added. Physical site modifications, such as removal of fill soil and the installation of French drains, were tested. The modification of irrigation cycles was also tested to increase the amount of air in the root zone.
Results and conclusions
Roots and soil of all symptomatic trees, as well as 30 percent of non-symptomatic trees, readily yielded isolates of Phytophthora species, including P. cinnamomi, and Pythium species. Symptomatic trees were usually located in areas where there was frequent standing water or where there was exposure to saline or alkali water. In addition, trees with earth fill over the root zone were likely to be symptomatic as were trees injured during construction. The first visible symptom was a generalized slight yellowing of the leaves. This persisted for as little as a few months or for as much as several years. This chlorotic stage was replaced by a transition period in which the color rapidly changed to an increased yellowing or a mottling of yellow and green colors. After several weeks the leaves rapidly turned orange and then brown, but remained attached as the tree died. Soil injections with ethazol provided symptom remission on affected trees, with the beneficial effects lasting at least 9 months, especially where there was less than 15 cm of soil fill. Soil injections with metalaxyl gave symptom remission in a higher percentage of cases, but the effects seemed to be shorter-lived (36 months) than with ethazol. The greening of leaves and symptom remission resulted from fungicide treatment, but the application of magnesium and zinc sulfates and nitrogen turned the leaves green within a day ortwo and provided an indication of chemical uptake. Physical site modification, such as removal of soil fill extended the time that the control was effective. Changes in drainage were also generally effective and economical, as well as changing the irrigation practices of the home owner to less frequent and deeper watering.
1. Campbell WA, 1948. Plant Disease Reporter 33,134-5.
2. Shew HD, Benson DM, 1982. Phytopathology 72,1029-32.