Posts Tagged ‘19:12’

Heat Damage to Vegetable Transplants

Friday, June 10th, 2011

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Watch for heat damage in transplants on black plastic mulch planted recently. This is a common problem in later plantings of peppers and tomatoes in particular.

Vegetable transplants are exposed to high soil temperatures at the soil line around the transplant hole. I took soil surface temperatures in black, white and metalized plastic mulch in mid-afternoon this Thursday (June 9). The air temperature was 97ºF. In open planting holes at the soil line, the temperature was >130ºF in the black plastic mulch, near 120ºF in the white plastic, and less than 110ºF in the metalized (aluminum) mulch.

The stem tissue just at or above the level of the plastic will be killed at these high temperatures and the transplants will then collapse and die. Small transplants do not have the ability to dissipate heat around the stem as roots are not yet grown out into the soil and water uptake is limited. Another factor in heat damage is that there is little or no shading of the mulch with the leaves of small transplants.

In addition, high bed temperatures have the potential to damage roots.

There are a number of practices that can reduce heat damage in later planted vegetable transplants:

● Delay planting until a cold front goes through if possible.

● Use larger transplants with greater stem diameters and more leaves to shade.

● Make a larger planting hole, cutting or burning out the plastic.

● When transplanting into the plastic, make sure the stems of transplants do not touch the plastic once set.

● Water sufficiently in the hole to reduce heat load, the more water the better.

● Plant in the evening, once the plastic has cooled down, or in the very early morning.

● Switch to white or aluminized plastic mulch for later plantings. This will reduce the heat loading significantly.

● In smaller plantings you may paint the planting zone on the black plastic mulch white with latex paint and then plant through this white strip once dry. You can also mulch around the planting holes with wet straw to reduce heat loading.

● Use overhead irrigation after planting to keep the plastic cooler.

 

Grower’s Guide to Understanding the DMI or SI Fungicides

Friday, June 10th, 2011

Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu

The following article by Andy Wyenandt, Ph.D., Specialist in Vegetable Pathology at Rutgers University is particularly timely since many vegetable growers are spraying fungicides to prevent many foliar diseases at this time of year especially on vine crops where DMI or SI fungicides are used very frequently.

The DMI (DeMethylation Inhibitors) or Sterol biosynthesis Inhibiting (SI’s) fungicides belong to FRAC code 3, which include the triazoles and imidazoles. Some of these fungicides are commonly known as Tilt (propiconazole), Rally (myclobutanil), Folicur (tebuconazole), and Procure (triflumizole). SIs work by inhibiting the biosynthesis of ergosterol, which is a major component of the plasma membrane of certain fungi and needed for fungal growth.

Resistance by fungi to the SI fungicides has been characterized and is generally known to be controlled by the accumulation of several independent mutations, or what is known as ‘continuous selection’ or ‘shifting’, in the fungus. Such that, in any given field population the sensitivity to the SI fungicide by the fungus may range from extremely high (highly sensitive, i.e. will be controlled by fungicide) to moderate (partially sensitive) or low (mostly resistant to fungicide). This type of resistance is also known as quantitative resistance. With quantitative resistance there are different levels of resistance to the fungicide due to independent mutations, which is unlike the target mutations that occur in qualitative resistance associated with the QoI fungicides (FRAC code 11).

Because different levels of resistance to the SI fungicide may exist in the field, the fungal population may behave differently to different rates of the SI fungicide being applied. If that is the case, it is suggested that using a higher rate of a SI fungicide, may improve control when lower rates have failed. For example, let’s say that a powdery mildew population on pumpkin has 25% high, 50% moderate, and 25% low sensitivity to a SI fungicide. If fungicide is applied at the low rate, only 25% of the population (highly sensitive) may be controlled. Whereas, if the high rate was used 75% of population may have been controlled.

The main point is that if low rates of SI fungicides have been used and control seems to be weakening, bumping to a higher rate may improve control. Unfortunately, it is difficult to determine what proportion of the powdery mildew population is sensitive or not sensitive by looking at the field until you have begun spraying. The best advice, if you are using low rates and think those rates are not working like you feel the rate should be bumped up to the high rate the next time the fungicide is sprayed, and if the high rate doesn’t work, it may be safe to assume the fungal population has grown mostly resistant. Importantly, if the high rate fails, whether you bumped up to a high rate or started with one, and control does not seem adequate, do not continue to use the fungicide.

Recognizing if and when fungicide chemistries are failing and when fungicide resistance is developing is critical to producing successful crops and why scouting on a regular basis, at least before and after each fungicide application, is important. Regular scouting can help reduce unwarranted and ineffective fungicide applications and help reduce wasted costs. Remember to always tank mix SI fungicides with protectant (M) fungicides (i.e., chlorothalonil) to help reduce the chances for fungicide resistance developing. Always apply SI fungicides according to label rates and resistant management recommendations and always be aware of the fungicide rates you are applying.

 

Results of the 2010 On-Farm Cucurbit Powdery Mildew Fungicide Resistance Trial

Friday, June 10th, 2011

Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu

In 2010 a multi-state project including DE, PA, MD, VA, NY and NJ was undertaken to determine the extent of resistance to fungicides for powdery mildew control in cucurbits. Plants of a susceptible pumpkin variety were grown to the 2-3 leaf stage, sprayed with different fungicides at varying rates and placed in commercial cucurbit fields with powdery mildew for 1-2 days. The plants were collected and allowed to continue to grow and develop symptoms in a greenhouse, then evaluated for the amount of powdery mildew on the leaves compared to the untreated plants exposed at the same time. The results from the two fields in DE showed that at least for these two fields that there is resistance in the powdery mildew populations to Topsin M, Flint, Endura, and Rally. The resistance to Rally at the high rate was very low indicating that some control would be expected at the highest label rate of Rally. No resistance was detected to Quintec, either in DE or any other state. I did not include Inspire or Folicur. No resistance was found to Inspire, but NY and PA did see some resistance to tebuconazole (Folicur). Cucurbit growers in DE and MD should not expect to see control of powdery mildew from thiophanate-methyl (TopsinM), or a stand-alone strobilurin fungicide like Flint, Quadris or Cabrio. The results of this trial indicate that the high rate of Pristine (Endura plus Cabrio) may still provide some control but there is resistance to the boscalid (Endura) component occurring in the region.

 

Vegetable Crop Insects – June 10, 2011

Friday, June 10th, 2011

Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu

Cucumbers
Cucumber beetles continue to be active so be sure to scout for beetles as well as aphids. Fresh market cucumbers are susceptible to bacterial wilt, so treatments should be applied before beetles feed extensively on cotyledons and the first true leaves. Although pickling cucumbers have a tolerance to wilt, a treatment may still be needed for machine-harvested pickling cucumbers when 5% of plants are infested with beetles and/or plants are showing fresh feeding injury. A treatment should be applied for aphids if 10 to 20 percent of the plants are infested with aphids with 5 or more aphids per leaf.

Melons
Continue to scout all melons for aphids, cucumber beetles, and spider mites. Although aphid populations are still relatively low, populations can quickly explode. The treatment threshold for aphids is 20% infested plants with at least 5 aphids per leaf. We continue to find fields with spider mites at economic levels. The threshold for mites is 20-30% infested crowns with 1-2 mites per leaf. Cucumber beetles continue to be an economic problem. Since beetles can continue to re-infest fields as well as hide under the plastic, be sure to check carefully for beetles as well as their feeding damage. Multiple applications are often needed to achieve effective control. Now that most fields are blooming, it is important to consider pollinators when making an insecticide application: http://extension.oregonstate.edu/catalog/pdf/pnw/pnw591.pdf.

Peppers
As soon as the first flowers can be found, be sure to consider a corn borer treatment. Depending on local corn borer trap catches, sprays should be applied on a 7 to 10-day schedule once pepper fruit is ¼ – ½ inch in diameter. Be sure to check local moth catches in your area by calling the Crop Pest Hotline (in state: 800-345-7544; out of state: 302-831-8851) or visiting our website at (http://ag.udel.edu/extension/IPM/traps/latestblt.html). You should also watch for an increase in aphid populations. A treatment may be needed prior to fruit set if you find 1-2 aphids per leaf for at least 2 consecutive weeks and beneficial activity is low.

Potatoes
Continue to scout fields for Colorado potato beetle (CPB) and leafhoppers. Adult CPB as well as the small and large larvae can now be found. A treatment should be considered for adults when you find 25 beetles per 50 plants and defoliation has reached the 10% level. Once CPB larvae are detected, the threshold is 4 small larvae per plant or 1.5 large larvae per plant. As a general guideline, controls should be applied for leafhoppers if you find ½ to one adult per sweep and/or one nymph per every 10 leaves.

Snap Beans
Continue to sample all seedling stage fields for leafhopper and thrips activity. The thrips threshold is 5-6 per leaflet and the leafhopper threshold is 5 per sweep. If both insects are present, the threshold for each should be reduced by on third. In addition, continue to watch for bean leaf beetle. Damage appears as circular holes in leaves and significant defoliation can quickly occur. As a general guideline, a treatment should be considered if defoliation exceeds 20% prebloom. As a general guideline, once corn borer catches reach 2 per night, fresh market and processing snap beans in the bud to pin stages should be sprayed for corn borer. Sprays will be needed at the bud and pin stages on processing beans. After the pin spray on processing beans, the spray schedule will be determined by a combination of both moth catches and field scouting. With the recent hot weather, beetles in the traps have made moth id more difficult:
http://ag.udel.edu/extension/IPM/traps/latestblt.html
and
http://ag.udel.edu/extension/IPM/thresh/snapbeanecbthresh.html.

Once pins are present on fresh market snap beans and corn borer trap catches are above 2 per night, a 7 to 10-day schedule should be maintained for corn borer control.

Sweet Corn
Continue to sample seedling stage fields for cutworms and flea beetles. You should also sample whorl through pre-tassel stage corn for corn borers and corn earworms. A treatment should be applied if 15% of the plants are infested with larvae. The first silk sprays will be needed for corn earworm as soon as ear shanks are visible. Be sure to check both black light and pheromone trap catches since the spray schedules can quickly change. With the recent hot weather, beetles in the light traps have made moth id more difficult. Trap catches are generally updated on Tuesday and Friday mornings (http://ag.udel.edu/extension/IPM/traps/latestblt.html and http://ag.udel.edu/extension/IPM/thresh/silkspraythresh.html). You can also call the Crop Pest Hotline for the most recent trap catches (in state: 800-345-7544; out of state: 302-831-8851).