Is rhubarb and asparagus that has been frosted safe to eat? Let’s look at each of these individually.
Rhubarb: A light frost will not harm rhubarb.  However, if temperatures were cold enough to cause the rhubarb leaves to wilt or become limp then damage has been done and such leaves should be removed and discarded.  Any new leaves that appear and are normal can be eaten.
Remember that the leaf blade of rhubarb is poisonous regardless of whether it suffered cold damage as it naturally contains oxalic acid.  The leaf stalk is the edible portion of this plant.  However, when leaves become frozen, the oxalic content of the stalks increases, making them dangerous to consume.  You can find more information on rhubarb at: http://www.bookstore.ksre.ksu.edu/pubs/ep99.pdf
Asparagus: Asparagus does not contain poisonous substances but frost will cause the spear tips to wilt and give them an off flavor.  Remove and discard any spears that show such damage. We also have a guide sheet on asparagus at: http://www.bookstore.ksre.ksu.edu/pubs/mf319.pdf  (Ward Upham)  

Sweet corn is one of those crops that is only "good" for a few days. If you want longer periods of production, consider staggering the planting. In other words, plant a small block, wait a period of time, and then plant the next block. Though it is tempting to follow a calendar schedule, such as planting a small block every week, it is better to use crop development as a trigger. If you plant on a calendar schedule, you may have noticed that later plantings often catch up with earlier ones. Instead, plant the next block of sweet corn when the previous one is one-half to one inch tall. (Ward Upham)

A common disease of roses is blackspot, a fungus disease that can cause defoliation of susceptible plants. Look for dark, circular lesions with feathery edges on the top surface of the leaves and raised purple spots on young canes. Infected leaves will often yellow between spots and eventually drop. The infection usually starts on the lower leaves and works its way up the plant. Blackspot is most severe under conditions of high relative humidity (>85%), warm temperatures (75 to 85 degrees F) and six or more hours of leaf wetness. Newly expanding leaves are most vulnerable to infection.
The fungus can survive on fallen leaves or canes and is disseminated primarily by splashing water.  Cultural practices are the first line of defense.
1. Don't plant susceptible roses unless you are willing to use fungicide sprays. For a list of blackspot resistant varieties, go to: https://ag.purdue.edu/btny/ppdl/Pages/POTW_old/3-22-04.html
2. Keep irrigation water off the foliage. Drip irrigation works well with roses.
3. Plant roses in sun in areas with good air movement to limit the amount of time the foliage is wet.
4. Remove diseased leaves that have fallen and prune out infected rose canes to minimize inoculum.
If needed, protect foliage with a regular spray program (10- to 14-day schedule) of effective fungicides. Recommended fungicides include tebuconazole (Bayer Disease Control for Roses, Flowers and Shrubs), myclobutanil (Immunox, Immunox Plus, F-Stop Lawn & Garden Fungicide), triticonazole (Ortho Rose & Flower Disease Control) and chlorothalonil (Broad Spectrum Lawn & Garden Fungicide, Garden Disease Control, others). (Ward Upham)

The most common fungicide used in fruit tree sprays is captan. Unfortunately, this product is subject to alkaline hydrolysis. This is a process whereby certain pesticides will break down when mixed with high pH water. So let’s say you mix up your spray mixture by captan to 5 gallons of water. If that water has a pH of 7, the captan will break down so that only half of it will still be present in 8 hours.  However, if the water you use has a pH of 10, half the captan will break down in 2 minutes. 
Malathion used to be the most common insecticide used for fruit pest control by gardeners but is becoming more difficult to find.  It isn’t nearly as sensitive to alkaline hydrolysis as captan but still will break down under high pH conditions.  Fortunately, it is stable at a pH between 5 and 7. Lambda-cyhalothrin, found in Bonide Fruit Tree & Plant Guard, is a relatively new product for fruit pest control that also is stable between a pH of 5 to 7.
Note that alkaline hydrolysis does not affect all pesticides.  Captan is the exception, not the rule. For a listing of common pesticides and their susceptibility to alkaline hydrolysis, see http://ecommons.library.cornell.edu/bitstream/1813/5149/1/FLS-118.pdf
So, how do you bring down the pH of your spray water if it is high? Commercial people use buffering agents but that may be difficult for homeowners to find. Food grade citric acid can help. If you have a pH of 8.0, add 2 ounces of this citric acid per 100 gallons of water (1 and 1/4 teaspoons per 10 gallons) to bring the pH down to about 5.5. (Ward Upham)

Peach leaf curl is a fungus disease that causes developing peach leaves to become puckered and distorted and show a reddish-green hue. A similar disease called plum pocket may develop on American and sand hill plums. Plum pocket results in formation of distorted, light green, bladder-shaped fruit. Asian and European plums are not susceptible to the local strain of plum pocket. Unfortunately, it is too late to control any of these diseases with fungicides this year.
Trees that are severely infected with peach leaf curl are likely to lose many leaves. If trees are healthy, new leaves will grow. Indicators of a healthy tree are large, deep green leaves and last year's growth being at least 18 to 24 inches long. If these tree vigor indicators are not present, especially if there was only 12 inches or less of growth last year, then a fertilizer application would be helpful.
The fertilizer should be spread on the soil under the branch area. Apply 1 and 1/3 to 2 cups of a 13-13-13 fertilizer under the branch area. If a soil test indicates that only nitrogen is needed, use 1/3 to 1½ cups of nitrate of soda (16-0-0) instead of the 13-13-13. You may also substitute a high nitrogen fertilizer such as a 27-3-4, 30-5-4 or something similar for the 13-13-13, but use only half the amount used for nitrate of soda. The sooner fertilizer is applied, the more immediate benefit it will have in promoting new leaf growth. Both peach leaf curl and plum pocket can be controlled with a single fungicide application applied this fall after leaf drop or early next spring before bud swell.
Effective fungicides include Bordeaux mixture and chlorothalonil (Bravo, Daconil and others). Be sure to cover the entire tree including the bark and trunk. (Ward Upham)

May is an excellent time to fertilize cool-season lawns such as tall fescue and Kentucky bluegrass if they will be irrigated throughout the summer. Non-irrigated lawns often go through a period of summer dormancy because of drought and do not need this fertilization.
May is a good time to fertilize because the springtime flush of growth characteristic of these grasses has tapered off, so the fertilizer you apply will be less likely to cause excessive shoot growth than if you fertilized at a full rate in April. Slow-release nitrogen sources are ideal. These nitrogen sources promote controlled growth, which is desirable as the stressful summer weather approaches. Relatively few fertilizers available to the homeowner supply ALL of the nitrogen in the slowly available form. But one such product that is widely available is Milorganite. Other such products available in the retail market include cottonseed meal, alfalfa-based fertilizers, and any other products derived from plants or animals. (Bloodmeal is an exception, and contrary to popular belief, the nitrogen it supplies is quickly available.) These products are all examples of natural organic fertilizers. They typically contain less than 10 percent nitrogen by weight, so compared to most synthetic fertilizers, more product must be applied to get the same amount of nitrogen. Translation: they are more expensive!
Apply enough to give the lawn one pound of nitrogen per 1,000 square feet. For example, if the fertilizer is 6 percent nitrogen by weight, you will need to apply almost 17 pounds of fertilizer product per 1,000 square feet. Summer lawn fertilizers that contain at least a portion of the nitrogen as slow-release are fine to use as well. Be sure to follow label directions. If cost is prohibitive, you can use the less expensive quick-release (i.e., soluble) sources, but split the application into two doses as follows: apply enough to give the lawn 0.5 lb nitrogen per 1,000 square feet in May and again in early June. (Ward Upham)

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