Compiled by Vern Grubinger, University of Vermont Extension
(802) 257-7967 or

TIME TO SIDEDRESS NITROGEN? (Adapted from UMaine Extension)

Heavy rains in some locations have probably leached much of the nitrogen from the root zone that was applied at or before planting. As a result, it would be appropriate to apply some nitrogen to sweet corn (30-50 lb./A) that is now at the late whorl to pre-tassel stages.  Due to the warm weather and resulting jump in growth, many fields are at the stage where they should be side-dressed anyway. Other crops such as tomato and winter squash may also benefit from side dressed N at this time. On tomato, use calcium nitrate or other nitrate forms of N because using ammonium N such as urea can increase the chance of blossom end rot.

WHEN TO SPRAY FOR APHIDS (Adapted from UMass Extension)

Besides spreading virus diseases, aphids in high numbers can cause leaf curling and yellowing, or deposits of honeydew on leaves or fruit or husks that can affect crop quality or yield. For this reason it can be important to manage aphids even if virus is not a concern. However, beneficial insects such as lady beetles, spined soldier bugs, insidious flower bugs, spiders, lacewings, and parasitic wasps often keep aphid  numbers low enough to prevent significant damage. Beneficial insects move around a lot, and build up wherever they find aphids to feed on. Early sprays targeting aphids may actually result in further aphid outbreaks, because the natural enemies that keep them in check are killed.

Scout your fields to see if aphids are present and in what number, then check back in a few days to see if the numbers are increasing or decreasing. Note which natural enemies are present  It is not unusual for aphid numbers to decrease even though no insecticide has been applied because the number of  natural enemies is building up. Check the undersides of leaves, including lower and mid level leaves. The following thresholds can be used to determine if insecticides are needed (sampling routine in parenthesis): Pumpkin and winter squash: 20% of leaves have more than 10 aphids (based on 50 leaves).  Pepper: 10 aphids per leaf (based on 4 leaves per plant, 25 plants). Tomato: 6 aphids per leaf (based on 2 leaves per plant, 25 plants).  Potato: 4 to 10 aphids per leaf (based on 25 to 50 compound leaves; higher threshold near harvest).  Sweet corn: 50% of plants have more than 50 aphids at emerging tassel stage (based on 100 plants).

LATE and EARLY BLIGHT (adapted from MA, NY and WI extension)

Late blight is much more destructive to potatoes and tomatoes than early blight. Early blight can be expected to occur every year on tomato, and occasionally on potato. It generally starts on older leaves and is easily identified by characteristic ‘rings’ inside the areas of dead tissue. Most conventional fungicide programs do a reasonably good job in managing early blight. On the other hand, late blight has occurred only occasionally in Vermont over the past several years, and to control it requires careful selection of fungicides and rigorous scouting to catch it early. Late blight moves in very quickly and can be very destructive. Rainy weather and temperatures between 65 and 70 degrees are ideal for late blight epidemics. The late blight fungus is inactive above 85 degrees.

This year, late blight has been confirmed in states to our south and west. While sanitation is key to preventing the disease from over-wintering on crop residues on your farm, the spores can travel a long way on wind, and many farms can become infected quickly. We saw this last year on tomato crops across southern Vermont. Protective fungicide sprays are the only way to prevent this disease from spreading once it arrives. Scout your fields frequently for late blight, particularly looking in low areas where water has collected or humidity is high. Wheel tracks are a good source of high humidity in the field as are areas along the tree line. Look for large black spots on the leave the size of a silver dollar. Young lesions appear as small necrotic (dead) spots that are collapsed and surrounded by light green tissue. These lesions are difficult to find if they are buried in the canopy. Under moist conditions a ring of white mildew appears at the margins of necrotic tissue, usually on the undersides of the leaf lesions. Lesions are also common on stems and leaf petioles where they girdle and blacken succulent tissues.

For late blight on potatoes, the fungicides Curzate, Acrobat MZ and Previcur Flex can be used.  These materials are not registered for tomato. Ridomil was the material of choice for late blight but many strains are now resistant to the fungicide.  For late blight on tomato fungicide options are Ridomil Gold MZ, Ridomil/Bravo, Bravo and Manzate.  Ridomil Gold MZ is formulated with Manzate. The rates of Ridomil Gold MZ and Ridomil/Bravo deliver about the same amounts of Bravo and Manzate that you would have if you were to use these products alone. Organic growers can use copper fungicides to get some protection of plant tissue from both early and late blight infection.


There have been reports of severe damage to strawberries from angular leaf spot. This disease is caused by a bacterium and it appears initially as angular lesions on the leaves that often look shiny like fingernail polish, but more importantly it causes the berries to have a blackened calyx, reducing their marketability. The disease is worse after springs with cool, wet weather. In fungicide efficacy trials over the past two years conducted by Dr. Schilder at Michigan State Univ., the best control has been with Kocide 2000 applied from 1 to 2 weeks before bloom every 7 to10 days until a week or so before the first harvest. That usually makes about 5 sprays total. Slight phytotoxicity may be observed as reddening of purpling of leaves. Please let me know if you believe you have had angular leaf spot in your berries, as I am trying to determine the extent this problem in Vermont.

STRAWBERRY RENOVATION (from Purdue University)

Matted row strawberry plantings must be renovated after harvest to establish new crowns for next year's crop. For best results, renovation should be started immediately after the harvest is completed to promote early runner formation. The earlier a runner gets set, the higher its yield potential. Renovation should be completed by mid-July in normal years. The following steps describe renovation of commercial conventional strawberry fields. (ed note: Most organic growers have found that renovation is practical only if weed pressure is not severe, and the cost of hand labor for weed control can be recovered through a reasonably high retail price. Except for the weed control sections below, the rest applies to organic production, too.)

Weed control: Annual broadleaf weeds can be controlled with 2,4-D amine formulations. Check the label, as only a few products are labeled for use on strawberries ( Formula 40 or Amine 4 applied immediately after final harvest. Be extremely careful to avoid drift when applying 2,4-D.  Even though the amine formulation is not highly volatile, it can volatilize under hot, humid conditions and can cause damage to desirable plants a considerable distance from the site of application. Some damage to strawberries is also possible. Understand the label completely before applying 2,4-D amine. If grasses are a problem, sethoxydim (Poast) will control annual and some perennial grasses. However, do not tank mix Poast and 2,4-D. (See the 2000-2002 New England Small Fruit Pest management Guide and the product label for rates and  precautions.)

Mow the old leaves off just above the crowns 3 to 5 days after herbicide application (or if not spraying, immediately after harvest is complete.)  Do not mow so low as to damage the crowns.

Fertilize the planting.  A soil test will help determine phosphorus and potassium needs, but foliar analysis is a more reliable measure of plant nutrition. For foliar analysis, sample the first fully expanded leaves following renovation. Nitrogen should be applied at 25 to 60 lbs/acre, depending on vigor. It is more efficient to split nitrogen applications into 2 or 3 applications at regular intervals, rather than apply it all at once. A good plan is to apply about half at renovation and half again in late August.

Subsoil: Where picker traffic has been heavy on wet soils, compaction may be severe. Subsoiling between rows will help break up compacted layers and provide better infiltration of water.  Subsoiling may be done later in the sequence if soils are too wet now.

Narrow the rows: Reduce the width of rows to a manageable width based on your row spacing, the aisle width desired, and the earliness of renovation. A desirable final row width to attain at the end of the season is 12 to18 inches. Wider rows lead to low productivity and increased disease pressure. This means that rows can be narrowed to as little as 6 inches during renovation. Use a rototiller or cultivator to achieve the reduction. Since more berries are produced at row edges than in the middle, narrow rows are superior to wide rows. Narrow rows will give better sunlight penetration, disease control, and  fruit quality.

Cultivate: Work in straw between rows and throw a small amount of soil over the row by cultivation. Strawberry crowns continue development at the top, and new roots are initiated above old roots on the crown, so ½ to 1 inch of soil on the crowns will facilitate rooting. This also helps provide a good rooting medium for the new runner plants.

Weed control: Pre-emergence weed control should begin immediately. Dacthal, Sinbar, or Devrinol are suggested materials. (see the 2000-2002 New England Small Fruit Pest Management Guide) and check the product labels carefully. Devrinol must be incorporated by irrigation, rainfall, or cultivation to be effective. Rate and timing of Sinbar application is critical.  If regrowth has started at all, significant damage may result. Some varieties are more sensitive to Sinbar than others.  If unsure, make a test application to a small area before treating the entire planting. Use 2 to 6 oz/acre/application and no more than 8 oz/acre/year total.  Sinbar should not be used on soils with low organic matter, or on sensitive varieties like Guardian, Darrow, Tribute, Tristar and possibly Honeoye. If Sinbar gets onto strawberry leaves, irrigate to wash it off.

Irrigate: Water is needed for both activation of herbicides and for plant growth. Don’t let the plants go into stress. The planting should receive 1 to 1-1/2 inches of water per week from either rain or irrigation.

Cultivate to sweep runners into the row until plant stand is sufficient. Thereafter, or in any case after September, any runner plant not yet rooted is not likely to produce fruit next year and is essentially a weed and should be removed. Coulter wheels and/or cultivators will help remove these excess plants in the aisles.

Adequate moisture and fertility during August and September will increase fruit bud formation and improve fruit yield for the coming year.  Continue irrigation through this time period and fertilize if necessary. An additional 20 to 30 pounds of N per acre is suggested, depending on the vigor.


Strawberries grown on plasticulture can be renovated in July. For varieties (Sweet Charlie) and plantings with moderate vigor, mow tops with a rotary mower, leaving several leaves on the plant. For very vigorous varieties (Chandler) and plantings, cutting away a portion of the crown with an asparagus knife leaving 3 crowns or a combination of mowing followed by crown thinning, may be the most effective renovation technique. After renovation, maintain adequate soil moisture and good insect and disease control. In early September, apply 60 pounds of N, P2O5, and K2O via drip irrigation and manage the renovated planting using the same cultural practices as for a new planting. Renovation has improved berry size, however, size is usually smaller than in the first harvest season. Marketable yields of renovated strawberries have been equal to yields in the first harvest season. Renovation is especially useful if the planting will be harvested as a P-Y-O.


 In the last issue it should have been stated that Chandler is a patented variety thus it is illegal to produce your own plugs from runner tips unless you pay royalties. Contact the nursery that sold you the original plants for more information.

(Mention of pesticides is for information purposes only; no endorsement of materials or brands is intended. Always read and follow instructions on the label.)