Did your evergreen plant start to turn yellow in the spring? The usual time of seasonal fall of needles is autumn and this is a natural process. Therefore, you need to pay attention to who or what is the cause of yellowing – it may be symptoms of pests or cultural problems. Several pests are known to feed on pine needles.
For some species, such as white pine and Scots pine, yellowing of older needles is a natural phenomenon. Usually pine holds needles for only one or two years. When in summer and autumn, the fall of the needle occurs over a long period of time, and it becomes more noticeable – this is a cause for concern.
Other conifers, such as the Fraser Fir, the Concolor Fir, the Douglas Fir, and others, hold their needles for five or more years, so in late September or October, internal needles rarely fall off. This may be a natural process of loss of the oldest internal needles, but it can also be a symptom caused by several pests or other factors. Usually, the main culprits are either spider mite damage, needle diseases, nutrient deficiencies or tree stress symptoms, such as heat (prolonged high temperatures) or drought (long periods of lack of soil moisture).
Here are some things to look for to help you diagnose what’s going on in your area.
The needles will show the characteristic “dotted”. When mites feed, they release the cell from the green chlorophyll. This results in yellowish spots along the needles.
Diseases of the needle:
Diseases caused by Rhizosphaera or Stigminia / Mycosphaerella on spruce and Swiss needle on fir. Using a hand lens, we see black fruiting bodies protruding from the stomata of infected needles. On a healthy needle the stomata are white.
Lack of nutrients:
Symptoms of nutrient deficiency can be anything from pale green to chlorosis of the needle (yellowing) of the needle tips and the whole needle. Look for other signs of defects and samples of damage throughout the site.
Stress – thermal or water:
The fall of leaves or needles is a common reaction of a tree to heat or drought. Needles can dry out during long dry or hot periods in late August and September, turn yellowish-brown, but remain on the trees until autumn, and then come off and fall during processing.
Maybe it’s a spider mite or a needle fungus, you need to develop a plan to control these pests next year.
In case of other possible stress, evaluate the area, and this fall take a sample of soil or leaves to determine if you need to adjust the fertilizer program.
Another pest of pine – leaf nematode, also called pine wilting nematode. Nematodes feed on plant tissues, causing the needles to turn yellow.
The pine nematode Bursaphelenchus xylophilus kills conifers, especially pines, many species. Scots pine is most often affected by the host. The nematode is transmitted by sawfly beetles, bark beetles, through their nutrient wounds into the resin channels. Here the nematode multiplies and impedes the movement of water through aquatic tissues, eventually causing the plant to wither.
Symptoms and diagnosis:
Early symptoms of pine wilting are often invisible or insignificant, alternating with rapid disappearance. The first visible symptoms usually include pale green and / or slight yellowing of the extremities. It can be obvious on one or more branches or can develop on all at once. In addition, within 48 hours after infection, there will be a visible loss of resin fluidity, which is observed on the cut branch. Trees often die so quickly that brown needles continue to cling to the branches. Sick pines usually die in early summer or early autumn, some species need 2 seasons.
Confirmation of the disease is carried out by detecting the pathogenic nematode under a microscope in the study of infected wood samples from the main trunk or affected branches.
Once the host plant has been infected with the pine nematode through the nutrition of the bark beetle, the nematode will feed and reproduce in the resin channels. When deciduous symptoms appear, a population of 1000 nematodes per gram of wood may occur. When stressors such as high temperatures, drought, or other pathogens increase, nematode populations increase. After the death of the host plant, the nematode turns into a latent form, and at this stage the larvae of the bark beetle, which feeds on the tree, can connect. This dispersed stage of the nematode migrates to the pupae of the bark beetle, then enters the bodies of newly formed adults of the bark beetle, which will look for other tree hosts, and repeat the life cycle of nematodes.
Pest control strategies:
– Do not leave an infected tree on the site. It is important to interrupt the life cycle of nematodes. Infected wood should be burned, can not be used as mulch to prevent the reproduction and overwintering of bark beetles in infected wood, and periods of high temperatures in summer contribute to epidemics, as well as periods of severe drought.
– Choose evergreen trees that are more resistant to pine nematodes. For example, blue, Norwegian and Serbian spruce, as well as oriental white pine. Avoid more susceptible species such as Scots pine, Austrian pine, Japanese black pine, red pine and long-leaved pine.
– Maintain the health of trees. Even a healthy tree can be infected with a pine forest nematode, and stressed trees are more attractive for bark beetle feeding. Arrange watering during long periods of drought.
– Periodically feed the trees in moderation.
– During the construction period it is important to prevent damage to the root system of trees.
– When driving on the site, protect the hole for the root system and trunks – all this reduces the likelihood of infection with butterflies and transmission of nematodes.
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