The plant is Photinia x fraseri. It is in the same plant family as Roses. This is likely a fungal disease called Powdery Mildew; one of the most common diseases of roses, peonies, phlox (and many other plants). Moderate temperatures of 60° to 80°F and shady conditions are the most favorable for powdery mildew development, and will spread. Roses do best in areas where there is full sun for 6-8 hours per day. The fungal spores are spread by wind. The good news is, you can manage this disease; sometimes fungicides are needed. Actinovate and Neem oil are two organic products that can help you with eliminating the fungus. Be sure to pick up any leaves and throw them out - do not put them in the compost pile. Also - do not water the leaves. Only water at the base of the plant.
It looks like Blackspot or similar fungus. Blackspot, "Diplocarpon rosae", is a nasty fungus that manifests itself on rose bushes as black spots on leaves progressing to black spots fringed with yellow rings on both sides of the leaves. As they develop the spots enlarge. Eventually, as the disease spreads, the entire leaves will go from green to yellow and then drop to the ground. With time the entire rose bush may become defoliated. Leaves less than two weeks old are the most susceptible to this disease.
The worst case scenario can be avoided with some preventative measures, a keen eye and diligence. While plants are dormant in spring, spray thoroughly with fungicidal soap and wettable sulphur (both readily available at the local plant nursery).
The white spots on your squash are probably powdery mildew. This fungus is common to squashes especially if the foliage is wet at night. Drier weather will help. The warm days and cool nights of late summer create an ideal climate for spore growth and dispersal.
Powdery mildew can be prevented, and it can be controlled once it appears, but it can't be cured. The key to preventing it is planting mildew-resistant or mildew- tolerant varieties. Resistant varieties get less mildew than susceptible varieties; tolerant varieties may get some mildew, but it shouldn't affect the performance of the plant. Prevention also includes siting plants where they will have good air circulation, and exposing as much leaf surface as possible to direct sunlight, which inhibits spore germination.
Environmental issues and disease often cause similar symptoms. For example, leaves dieing could be caused by too much or not enough water, and the fungal disease called Fusarium wilt causes similar symptoms. Usually the first symptom is a slight yellowing of lower leaves on just one side of the plant or one stem, or a slight wilting and drooping of the lower leaves. A distinct brown discoloration of the water and food channels in the stem can be seen by cutting a stem close to the base of the plant. Since soil retains this fungus indefinitely and is more prevelant in over-watered soils/plants its recommended that you don't re-plant tomatoes (or related plants) and other plants that are susceptible to this disease in the same spot, especially year after year.
This is not likely an infectious disease, but looks like typical high temperature damage, maybe in combination with a lack of water. Even if all the other plants in the hedge row look fine, it sometimes happens that one plant does not develop as healthy a root system as the other plants, or the soil in the immediate area around the roots of the affected plant is not holding sufficient amounts of water, even though we'd assume that the soil is the same as for the other plants; sometimes its just different enough to make the one plant grow differently, or suffer damage. We recommend using a soil probe (which can be made at home or purchased from a garden center) to check the moisture of the soil around the root zone, and water when necessary to keep the soil evenly moist.
Based on your description and the fact that it is a recurring condition, your apple tree may have Fire Blight. The primary sign of this bacterial infection is black “shepherd’s crook” twigs. There may also be light brown to blackened leaves. Here's a link for Apple Tree information from the Colorado Extension Agency. At the bottom of the page there are several links discussing apple diseases and pests including Fire Blight. You'll want to look at the images of branches affected by Fire Blight which will help you determine if it is present on your tree and what treatment options are available. If the Fire Blight description doesn't fit your tree's appearance, you can check out the other links for possibilities.
Enivornmental issues and disease often cause similar symptoms. For example, yellowing of leaves could be from too much water, and this fungal disease called Fusarium wilt causes similar symptoms. Usually the first symptom is a slight yellowing of lower leaves on just one side of the plant or one stem, or a slight wilting and drooping of the lower leaves. A distinct brown discoloration of the water and food channels in the stem can be seen by cutting a stem close to the base of the plant. Since soil retains this fungus indefinitely and is more prevelant in over-watered soils/plants its recommended that you don't re-plant tomatoes (or related plants) and other plants that are susceptible to this disease in the same spot, especially year after year.
Diseases are very challenging to identify from photographs, but this might be bacterial spot, a disease that affects foliage in humid, wet growing seasons. It starts as dark brown or black spots surrounded by yellow halos. On the edge of the leaf, turns it white and then brown, and spreads inward, usually on new growth. To minimize spread, water early in the day with drip irrigation. Overhead watering may spread the disease by splashing pathogens. If disease is not extensive, snip off affected foliage and dispose in the trash. When more leaves are affected, cut off as much of the damage as you can and place the pots farther apart. The disease can spread on clippers and other tools, so prepare a solution of 1:10 bleach:water and dip your clipper in it between plants.
Hard to say for sure but it may be excess salts (either from water or fertilizers), or it could be a mold growing in the organic matter, and finally it could be a sign of an insect called mealybug (or root mealy bug), but we doubt its the bug. Leaching the soil (running water thru it until you don't see the white stuff after the soil dries) will help remove salts, however, if its a fungus you'll want to get rid of it, and the easiest way to do that is to replace the soil and clean/scrub out the pot with bleach water before you use it again. To avoid this in future, avoid overwatering and move any empty pots or open bags of potting soil out of the elements to help keep it dry.
We had a look at your leaf and zoomed in as best we could. It could be scale sucking the nutrients from the stems and starving the leaves of moisture and nutrients, We can vaguely see white spots, and the main rib appears pronounced. So it could be a slow starvation causing the browning and brittle appearance.
However, there are most likely several other issues that need to be checked. Here are some websites with great information on lime tree care and diseases. The first website resource covers growing tips and problems and is very informative - http://citrus.myindoorgreenspot.com/icitrus.htm
The second website is from the University of California and has pictures of actual disease processes and insect damage - http://www.ipm.ucdavis.edu/PMG/C107/m107bpleaftwigdis.html