REPORT FROM THE UVM PLANT DIAGNOSTIC CLINIC
(Ann Hazelrigg, University of Vermont Extension)
With a fair amount of rain these past few weeks I suspect we will be seeing a lot more foliar fungal and bacterial diseases in crops. With the rain, fungal spores are splashed up from the soil where they have overwintered to the lower leaves of plants. When we have weather that allows for 6-8 hours of leaf wetness, either from dew or rain, the spore is able to penetrate healthy tissue (bacteria are different, they can only penetrate tissue where there is a wound or a natural opening) and start an infection. If you have used a mulch, some of this rain splash upwards in the plant can be limited.
As the fungus grows through the plant tissue, it feeds on cellular contents and death of these cells occurs and we see a brown spot. As we continue to get warm wet weather, fungi produce fruiting bodies (little microscopic mushrooms) that will produce more spores that are windblown or splashed upwards in the plant, producing more spots. If weather conditions are perfect, this cycle may only take 5-7 days. As a grower, anything you can do to improve air circulation and reduce the 6-8 hours of leaf wetness will decrease fungal infections. Good crop spacing, staking of plants, avoiding overhead watering, especially in the evening will all go a long way to minimizing disease.
If you apply a fungicide, this material does not kill the fungus but provides a barrier so the fungal spores are not able to penetrate the plant tissue. These fungicides have to be reapplied on a regular basis (every 7 days or so) due to weathering off of the material and the growth of new plant tissue that is unprotected.
Tomatoes. A lot of Septoria leaf spot (small spots with gray centers) and early blight (target shaped spot) will be showing up in the next few weeks. Keeping up with nitrogen fertility will help the plants produce new healthy tissue that can ‘out run’ some of the disease. Protect healthy tissue with fungicides.
Peppers. There have been lots of bacterial leaf spot in the past couple of years. Spots start out as water soaked areas, turning brown. The disease when severe can cause defoliation and spots on fruit. Bacterial diseases seem to be on the rise in our area. Not sure if it is a warming trend or whether seed is more infested with bacteria. Hot water treatment or Clorox treatment of seed will reduce the pathogen. This disease also can affect tomatoes.
Pumpkins, peppers. With these repeated downpours, many fields may have been saturated for 24 hours. These are perfect conditions for Phytophthora rot. Scout any areas of fields that are lower or have had poor drainage and standing water and watch for collapse of plants. The disease is hard to suppress once it starts in a field. If you have infected plants in one area of a field it is best to till under infected plants. The pathogen is moved easily from field to field so cleaning of equipment is extremely important.
Cucurbits. There have been lots of reports south of us seeing downy mildew in crops. So far, (although the latest storms may change this) we have not seen it in Vermont. This is one disease, like late blight in tomato and potato, that does not overwinter in Vermont but blows in from southern areas on storms. Watch for angular leafspots on the leaves delineated by veins. Spots are pale green then yellow before turning brown. Leaf undersides will show a purple gray fungus where the spot is. Leaf petioles will usually stay alive and upright after the leaf has died and drooped. Spores of the fungus are not always present and diagnosis can be difficult. Send to the clinic if you want confirmation.
Strawberries. We have seen a case of complete collapse of a field of plug plants that looked fine the previous fall. The crowns were dark and mealy and we were concerned it may be a southern disease called anthracnose, moving north. This disease can attack fruit, runners, petioles, and crowns. Dark elongated lesions develop on petioles and runner stems, which are sometimes girdled by lesions. In this case however the collapse was probably due to winter damage and black root rot complex, rather than anthracnose crown rot since there were few lesions on petioles.
Spinach. One grower lost an entire planting of spinach due to Fusarium. Foliage was yellow with wilting. After cutting open the root system, there was brown discoloration in the water conducting tissue indicating the disease. The grower will rotate and use resistant varieties next year. Anytime you see a problem (wilt or poor growth) in the above ground part of the plant, remember that half of that plant is underground. Check the root system to see if there are white healthy roots. Cut into the root longitudinally to see if the water conducting tissue is brown indicating a wilt disease.
USING PYGANIC FOR ORGANIC LEAFHOPPER CONTROL
(adapted from Abby Seaman, Cornell IPM program)
An efficacy trial including two products approved for organic production was conducted in research plots using the potato variety Superior. Treatments were sprayed 3 times at weekly intervals with a backpack sprayer. Treatments were:
Pyganic 1.4 EC (32 oz/A) - starting at adult arrival (June 21, June
30; July 7)
Pyganic 1.4 EC (32 oz/A) - starting at nymph hatch (July 12, July 19, July 25)
Pyganic + Surround (25 lb/A) - starting at adult arrival (June 21, June 30; July 7)
Pyganic + Surround (25 lb/A) - starting at nymph hatch (July 12, July 19, July 25)
PLH nymph populations were significantly reduced in both of the adult-arrival-timed treatments, and hopper-burn ratings were significantly reduced in all but the Pyganic + Surround at adult-arrival treatment. Yield was significantly increased in all treatments, compared with the untreated control, with no significant differences among treatments. The addition of Surround to Pyganic did not improve control, and time of initiation of control does not appear to be critical for improving yield. The average yield in the treated plots (297 cwt) compares favorably with the long-term average yield of Superior (285 cwt/A) in variety trials.
The average yield increase in the treated plots was 65 cwt/A, which at a wholesale price of $1.35/lb for organic potatoes increased income by approximately $900 per acre. The cost of three applications of Pyganic is approximately $165 per acre (material only). The variety "Superior" has shown moderate resistance to PLH in previous trials. Varieties more susceptible to PLH damage may need more than three applications of Pyganic to achieve optimum yield. We used a 32 oz/A rate of the 1.4% EC formulation of Pyganic. There is also a 5% EC formulation of Pyganic. An equivalent rate of the 5% formulation would be 9 oz/A.
(adapted from UMass Extension)
Late July and early August are usually the time when we see tomato hornworms, which typically appear in small numbers and cause impressive feeding damage to just a few plants. Larvae consume large amounts of foliage on peppers, tomatoes, eggplant, potatoes, and related weeds. Now is the time to scout; one often sees defoliated stalks or the dark-green droppings before the caterpillar is located.
Where damage is unacceptable, or if numbers are high, use a selective material that will conserve beneficial insects, such as a Bt product, because predators and parasites are likely keeping aphid populations under control. Although Bt usually works best on small larvae, in this case it will work very well even against large hornworms. In peppers, controls used for European corn borer should also control hornworms.
A parasitic Braconid wasp is an important and fairly common natural enemy of the hornworms. The wasps lay their eggs inside the body of the caterpillars. After feeding within the caterpillar body, the larvae of the wasps eat out through the skin and spin the cocoons on the caterpillar surface. The adult wasps later cut out circular lids and escape from the cocoons to attack other hornworms. If one is hand-picking hornworms, those with clusters of white cocoons of the parasitic wasp on their back should not be killed.
SCOUT FOR EUROPEAN CORN BORER
(adapted from John Mishanec, Cornell IPM)
The best time to scout for ECB is when a field is just in the spike stage. That is when the tassel is just beginning to poke out. Scout your corn fields and stop at 10 locations. At each location, inspect five plants for ECB damage. You are looking for small holes in the leaves and frass (saw dust from insect feeding) at the top of the plant. Keep a running count of how many you find. You will have looked at 50 plants total at 10 locations so just multiply the number of infested plants by two and you have the percentage of ECB in the field. If you are over 5%, than a control is called for.
For corn that is just coming into tassel, you will now find less ECB damage than in earlier planted fields. In eastern NY I am now finding about 10-15% corn borer infestation in the tassels. The best time to control ECB is when the tassel is just opening up and the individual tassels are still pointing up. Once the tassels are completely open, it is probably too late to get good control. ECB like it when it is cool and are out at night so spray in early morning to get best control, and avoid honey bees
Timing is very important. You have to wait till the tassel is open but still pointing vertical. When the tassels are horizontal, it is too late. Put your first spray down when you see around 30-40% of the field in early tassel. Wait 4-5 days for the rest of the field to come into tassel and make your second application. This will do a good job controlling the ECB larvae. Otherwise, the larvae will bore into the stem and be impossible to control. Remember, spike tassel is too early and full tassel is too late. You need to time your application just right. So look at your fields and keep track of when they are coming into tassel.
TWILIGHT MEETING 5:30 PM, JULY 31, EDGEWATER FARM
Edgewater Farm is a diversified farm growing fruits, vegetables, ornamentals, and cut flowers. They market their crops retail through a farm stand, wholesale, PYO, and through a CSA membership. This meeting highlights a variety of technologies used in crop production, including high tunnels and plasticulture, as well as crop rotation and cover cropping. For more info visit www.uvm.edu/vtvegandberry click on meetings, or call Seth Wilner at 603-863-9200.
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