Ornamental & Tree Diseases
Littleleaf Disease
Littleleaf disease, caused by the oomycetous fungus Phytophthora cinnamomi, is the most damaging disease of shortleaf pine in the Piedmont region of the South. Loblolly pine is affected, but to a lesser extent and usually in areas associated with infected shortleaf pine.
Phytophthora species are soil-borne water molds. They flourish under conditions of high moisture frequently associated with plastic, severely eroded soils with reduced moisture permeability. Their energy requirements are satisfied by infecting a plants feeder roots, those responsible for absorbing water and mineral elements from the soil.
Historically, Phytophthora species have been pathogens primarily on agricultural crops. An explanation for the presence of large populations of Phytophthora species in forest soils is that millions of acres of land currently growing trees was once used for agricultural purposes. This is certainly the case for littleleaf disease.
The history of littleleaf disease can be traced back to land clearing for agricultural use in the mid to late 1700's of portions of the Piedmont Plateau (Alabama through Virginia), the Appalachian Plateau (eastern Kentucky), and the Atlantic Coastal Plain (north-central Mississippi). Following land abuse by repeated planting of cotton and then land abandonment in the mid to late 1800's, natural regeneration of shortleaf and other southern pines occurred on these severely eroded, mineral depleted, Phytophthora-infested soils. By the early 1900's, shortleaf pines approximately 20 years old began to decline and die of a disease later described as littleleaf. An impact study conducted in the early 1950's revealed that littleleaf disease had resulted in the loss of more than 146 million board feet of sawtimber and was affecting trees growing on more than 15 million acres (35% of the commercial shortleaf range).
Littleleaf disease is most evident in shortleaf pine stands more than 20 years old, causing little to no damage in younger stands. Phytophthora cinnamomi overwinters in soil and infected roots as thick-walled resting spores (oospores). Survival of mycelium in infected roots is short-lived because of the suppressing effect of competitive saprophytic colonization by soil microorganisms. In the spring, oospores are the most significant type of primary inoculum if conditions are dry. If spring conditions are wet, zoospores are produced in large numbers. The littleleaf fungus is a common inhabitant of soils throughout most of the world. Therefore, long range dissemination is not as important a consideration as the soil population of the organism. Short distance spread occurs by swimming zoospores and by growth of the fungus following germination of zoospores and oospores. Entry into shortleaf pine roots occurs only through root tips or root hairs as a result of either direct penetration on wounds. Under conditions of high moisture and low fertility, infected shortleaf pines cannot replace root tips and root hairs as rapidly as they are destroyed, resulting in a number of symptoms.
The first observable symptom on trees infected with P. cinnamomi is death of root tips and root hairs. Rootlet loss results in the production of above-ground symptoms characteristic of littleleaf disease. Initial yellowing and thinning of crowns progress within 2 to 3 years to overall crown decline characterized by stunted shoot growth, tufts of chlorotic needles measuring only 1 to 2 cm in length (hence the name "littleleaf") at branch terminals, and crown dieback. This loss of a significant portion of photosynthetic activity in affected crowns accounts for a major reduction in radial growth. As the disease progresses, additional symptoms are produced including unusually heavy crops of persistent, small cones containing aborted or sterile seed and profuse sprouting at bases of stems and trunks. Tree death may occur within one year on poor sites or gradually decline over a period of 12 to 15 years on better sites. Most trees die within 6 years after the onset of visible aboveground symptoms.
Management recommendations include the following:
1. Evade P. cinnamomi by regenerating shortleaf and loblolly pine on low hazard sites (fertile sites with little to no erosion, friable subsoils with slight to no mottling, and depth to zone of reduced permeability more than 18 inches deep.
2. Plant loblolly on high hazard sites (mineral depleted, eroded sites with soils high in clay content and poor internal drainage). Also consider silvicultural practices that rehabilitate deteriorated soils, such as planting certain hardwoods or grass.
3. Where economics allow, apply mineral amendments. Current recommendations call for applying 5-10-5 (nitrogen-phosphorus-potassium) at the rate of 2117 lb/ac (2,373 kg/ha) with 405 lb/ac (454 kg/ha) of ammonium sulfate.
4. In severe littleleaf areas, manage shortleaf and loblolly pine for pulp.
5. Less than 10% infection, cut lightly at 10 year intervals; between 10-25% infection, cut every 6 years; and greater than 25% infection, cut all shortleaf pine when merchantable.