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The fungus produces two types of necrotic lesions on infected needles. The first type is initially straw yellow, becoming light brown with a dark border. The second type of lesion is a "bar spot" and consists of a brown spot on an amber yellow band. The yellow tissue is infiltrated with resin (Sinclair et al., 1987; Hansen and Lewis, 1997).
On needles of Pinus mugo symptoms first appear on needles as yellow to light orange, sometimes resin-soaked spots, which later become dark-brown in the centre with a yellow border (see Pictures). They usually enlarge to bands that encircle needles and cause death of parts beyond. Diseased needles typically show dead tips, central zones with spots in green tissue, and green bases (see Pictures) (Pehl and Wulf, 2001).
The transition from lesions to healthy green tissue is abrupt. In the brown-coloured dead parts of the needle, black stroma of the fructification develops under the epidermis visible as round black spots (see Pictures). During further development the oval-shaped fruit bodies, arranged parallel to the long axis of the needle, break through the epidermis opening by a longitudinal slit, or two slits, raising a flap of epidermis and hypodermal tissue (see Pictures). Under moist conditions, mature conidiomata produce mucilaginous, olive green spore masses (see Pictures). After severe attack the whole needle initially turns brown, then light brown to grey (see Pictures), and abscises prematurely. Less severe damage may delay needle fall for one or two years. Heavily infected pines typically show twigs with only last year's needles, giving a paintbrush appearance as the pathogen develops; these needles may shed. Over several years this may result in branch and tree death (Pehl and Wulf, 2001).
Often needles of lower branches are attacked by the brown-spot fungus and the pathogen gradually moves up the crown.
Cultural Control and Sanitary Methods
In longleaf pine stands brown-spot needle blight can be controlled by prescribed ground fires. Young pines in the grass stage can survive these low-intensity fires, which are used in winter to destroy dead infected foliage that harbors the pathogen (Sinclair et al., 1987). Another practical suppression method is shelterwood regeneration. This is the most promising approach to natural regeneration of longleaf pine in which seedlings are established as advanced reproduction under overstories of medium density. The pine canopy then protects the regenerated seedlings from brown-spot infection (Phelps et al., 1978).
Dense planting supports infection so it is important to keep enough space between seedlings, in both nurseries and plantations. While the foliage is wet, shearing and other cultural operations that could spread spores should be avoided (Sinclair et al., 1987). The grower should avoid planting his land with just one pine species or variety to prevent catastrophic losses (Phelps et al., 1978).
In Scots pine, long-needle varieties are somewhat resistant to infection and should be the preferred species in Christmas tree plantations. Varieties such as Austrian Hill or German should be planted to reduce brown-spot infections (Phelps et al., 1978).
Extensive experiments on spraying to control brown-spot in nurseries were conducted by Siggers (1932, 1944). The fact that conidia may be produced in any season throughout the year; that they are distributed by rain-splash; that ascospores are formed in dead tissues most on fallen needles and are air-borne; and that several crops of new needles may be formed within a year indicate that numerous sprayings would be required to give adequate protection.
According to Phelps et al. (1978), Skilling and Nicholls (1974) and Kais (1975b), brown spot needle blight is easily suppressed by applications of Bordeaux mixture, chlorothalonil, benomyl and copper hydroxide in nurseries, seed orchards, and plantations of longleaf pine and Scots pine. Seedlings should be sprayed at 10- to 30-day intervals depending on the amount of rainfall, from the beginning of spring through late summer. It is important to initiate spraying in the spring when the newly emerging fascicled needles are 2 to 5 cm long. Usually four to six applications are sufficient (Phelps et al., 1978). It is also recommended to make a final spray just prior to planting. This will ensure protection during establishment of seedlings in the field.
In Central America, M. dearnessii is omnipresent but rarely in the form of a serious needle blight. Often secondary needles are affected, causing some premature needle cast on hosts in natural pine stands. Severe needle disease can be observed on hosts at the extremes of their altitudinal range (Evans, 1984).
First reports of the brown-spot fungus as a serious needle blight pathogen are on Pinus palustris (longleaf pine) in the Gulf States of the USA. (Hedgcock, 1929). For Evans (1984) this is additional evidence to the hypothesis that the fungus is an exotic in this region, attacking susceptible and non-adapted indigenous pine species. Brown-spot blight continues to be the most important disease of Pinus palustris in the southern USA. This pathogen seems to be the main limiting factor to the establishment of this pine species in its natural range (Henry, 1954; Jewell, 1983). On P. ponderosa in Missouri, Luttrell (1949) associated a serious decline with brown spot needle blight. In several north-central states the brown-spot fungus is a major constraint to the growing of P. sylvestris for Christmas trees (Nicholls et al., 1973; Skilling and Nicholls, 1974) making the affected trees unsaleable. There are also reports from the Altiplano of Colombia that the pathogen is the cause of a severely debilitating needle cast of P. radiata (Gibson 1980).