I have three rose bushes in my front yard, one is doing great but two have yellow and black spots on the leaves and one which is particularly sad has a green spider on it (like the size of your regular house spider... maybe a little smaller but I don't think it is a mite) the spider appears to have made a web under the leaves. I don't know if there is a connection but I want to save these bushes. Any ideas on the problem and what I can do?!
If the "black spots" are round and bigger than tiny spots, then your rose is infected by a fungus called "Black Spots" (Diplocarpon rosae) - for obvious reason. The black spots appear while the leaves are still green. They then enlarge and get up to 1/2 an inch in diameter. Over a period of a few days, the leaves turn yellow, starting from the perimeter of the black spots. The yellowed leaves then drop off. Each black spotted lesion releases tens of thousands of the fungal spores, ready to infect other parts of the plant, or other plants nearby. They also survive through the winter in dead leaves, but not directly in the soil. The fungal spores are spread mainly by water splashes, but can also be wind borne. Contamination from fallen infected leaves, thererfore, affects the lower leaves initially, then spread by being splashed up to higher leaves by rain drops or overhead watering. The fungus requires a minimum period of continuous moisture on the surface of the leaves (7 hours), and a minimum temperature to grow (65 F). High temperatures tend to inhibit it (>85-90 F). Therefore, it tends to affect roses in the warmer summer months if it has been rainy or if over head watering is used frequently. Once infected, there are very few antifungal agents out in the market that will "cure" the disease. And none of these are available for sale to residential gardeners in Canada. The focus should bve on prevention. Given what we know of the source of the spores and the conditions required for the spores to germinate, the following preventative measures are recommended: 1. Rake and discard all fallen leaves (do not put these in the compost heap) 2. Also prune and discard any obviously infected canes/leaves 3. Avoid wetting the foliage especially during dark cloudy days or late in the evening - conditions that do not allow for rapid drying of surface moisture. 4. Grow plants in a sunny location and practise good pruning to keep the plants open - to promote rapid drying of the foliage. 5. Grow roses in situations which allow good air circulation. In addition, the susceptibility of the different varieties of roses vary, and the occurrence of the disease in those varieties may differ depending on the locations. It is worthwhile choosing rose varieties that are known to be more resistant in your area. You may be able to get that information form you local rose nurseries, observing how different varieties do in local rose gardens, from your local rose society, etc. The are some chemical sprays that are used to prevent the risk of the infestation taking hold. In Canada, most, if not all, of these chemicals are not available to residential gardeners. There are other common chemicals that are purported to be effected in prevention - lime sulphur, copper products, baking soda, etc.. Most sprays are contact agents - they need to be applied every 7-14 days. Spider mites, on the other hand, are tiny arachnids (some family as spiders and fleas). They are 8 legged, but extremely small - the largest may be 1/20th of an inch. To the naked eyes, they just look like specks of dust which are able to move around. They are fairly ubiquitous creatures in the garden, and affect a wide variety of plants, forming large destructive colonies very rapidly when the conditions are optimal. There are a number of different species around, but the most common culprits are species belonging to the Tetranychus genus. For all intents and purposes, gardeners don't really try to identify the species involved. They infest the underside of leaves in large colonies consisting of several hundred critters. If you hold a blank piece of paper under the affected leaf, then give the leaf a good shake, you should see some of the individuals as they drop on to the paper and start scurrying around. If you inspect the underside of the leaf, you should find a web made of strains that are finer than those you find on spider webs. The damage is done when these creatures suck the juices out of the cells of the leaves. Damage first shows up as yellow to pale very fine speckling on the upper surface of the leaf. As the infestation persist, the leaves turn pale, yellow and even bronze, then dry up and drop off. Their numbers increase rapidly under ideal conditions - good food supply, hot dusty environment. Unfortunately for us gardeners, water stressed plants that grow under these conditions also tend to suffer the most damage at the fastest rate. Minor infestations usually do not affect the overall health of the plant, but there is always the worry that even these minor infestations may develop into a life threatening infestation for the plant - given the right conditions. Here, again, chemical controls are usually not available to residential gardeners (in Canada), except for uncontrolled houshold products such as neem oil and soap. Given our knowledge of these creatures: 1. monitor your plants for the earliest signs of infestations - it's easier to control colonies when they are small. And it may also identify adverse cultural conditions affecting that plant that need to be corrected. 2. keep your plants well hydrated and the soil moist. Use mulching to avoid dry dusty soil surfaces. 3. if you see signs of infestation, spraying the undersides of the foliage with a forceful spray of water every day is the best method of keeping the infestation under control. Even if there are no signs of infection, the regular spraying when days are hot and dry may help to limit infestation (spray in the mornings, to reduce the risk of black spots). 4. if possible, avoid growing susceptible plants - in your case, roses - in locations which are expected to be hot and dry (e.g., by the side of the house, in a south/ southwest or west position). From your description, the spider like creature you found is actually a spider - it's far too big for a spider mite. But your observation of webbing on the underside of the leaf can still be a sign of spider mite infestation.
