Vermont Vegetable and Berry News – July 15, 2005
Compiled by Vern Grubinger
University of Vermont Extension
(802) 257-7967 ext.13

(Gary Pavlis, Rutgers Extension)

Farm visits have turned up a number of canes dying from what used to be called winter damage. We now recognize that this wilting and death of individual canes during the summer can also be due to Phomopsis. Under severe disease conditions, several canes may be affected on a single bush. This fungus overwinters in infected twigs and canes, and produces infective spores. The greatest number of spores are released during bloom and petal fall and enter twigs or canes through injury sites, particularly those caused by winter damage, mechanical harvesters or early spring frosts. Samples taken from canes confirmed Phomopsis at several farms, however, stem blight, Botryosphaeria, was also confirmed. Like Phomopsis, this fungus enters the plant through wounds and causes rapid death of individual canes and entire bushes. This disease is especially severe on 1 and 2 year old plants of susceptible cultivars. In the field, the most obvious symptom is called 'flagging', where stems recently killed by the fungus do not drop their leaves. Stem blight has recently been found most often in the 'Duke' variety.

Control of Phomopsis and Botryosphaeria depends largely on cultural methods. It is important to discourage late-season growth and promote early hardening off thus late-season fertilization, late-season weed cleanup and late-season irrigation should be avoided. Pruning to remove infected stems is the best method of reducing disease in established fields. Pruning serves two functions: 1) removes infections from bushes, preventing eventual death of the plant, and 2) reduces the number of spores released in the field by removing dead, spore bearing stems. Pruning can be done at any time infected stems are observed, but care should be taken to cut well below the infected area. After a stem is removed, examine the cut end of the remaining stem. If any brown areas are visible in this cross-section, a cut must be made further down the stem until all infected tissue is removed.

(Ruth Hazzard, UMass Extension)

Flea beetle numbers have gone down – it is likely that we are between adult generations – but pupae and larvae are in the soil, near the roots of early brassicas. Will you be planting fall broccoli or greens? Think twice before you put them close to your spring plantings – especially if you had a significant number of flea beetles. Those new adult beetles that will be emerging in late July and early August will be very hungry. If your fall brassicas are close by, feeding damage will be heavy. Try to rotate fall brassicas to a field that did not have spring brassicas.

(adapted from Michelle Infante-Casella, Rutgers Extension)

It’s not uncommon for pepper plants to flower but fail to set fruit. Flowers that do not pollinate properly may abort, creating a yield loss. If temperatures are too high during flowering, or if other adverse weather conditions occur, flowers may abort due to plant stress. Once flowers are pollinated and fruit begin to form the plant starts to use more water and nutrients, so irrigation and fertilization are key at this time. Also, fruit should be protected from insects and diseases that may make them unmarketable.

As fruit develops it creates stress on the plant because of the “source/sink” relationship in plants. When a plant begins to grow, the growing tips act as a “sink” that demands the most water and nutrients and is the primary growth area. The older leaves are the “source”, since they are actively photosynthesizing (producing energy) and would be first sacrificed if the plant were under environmental stress. This is why when a plant is under drought stress, the lower leaves may wilt and yellow first and fall off sooner, due to water and nutrients being diverted to other areas of the plant. When flowers or fruit form, they become the “sink” and the leaves, roots, and stems of the plant are their “source” of energy, water, and nutrients. Plants under stress will form flowers, and set fruit to make seeds in order to continue the next generation. This is a survival mechanism, and it explains why stressed, root-bound, sickly looking transplants left over in greenhouse trays will form fruit, even when the plant is very small. Therefore, if you remove pepper fruit, you remove the “sink” and allow energy, water, and nutrients back to produce more leaves, stems, flowers, and ultimately, more fruit.

Thus, timely harvests of bell peppers can help maintain production through the season, which can sometimes extend into fall with the help of row covers or mild weather conditions. Bell peppers are typically harvested at 10 to 15 day intervals for green peppers. If red peppers are desired due to higher prices or market demand, fruit must be left on the plant. From the time a green pepper reaches its final size it will take about 10 days for it to become fully red. When peppers are left on the plant to turn red, this adds more stress to the plant and physiologically promotes senescence (the gradual death process), so it makes sense to harvest for green fruit early in the season and only leave fruit to turn red after the plants have been harvested several times.

Peppers should be harvested during cooler parts of the day when field heat is minimal and quickly placed in cold storage to increase shelf life. Peppers are sensitive to chilling injury and are best stored at 45 to 55 degrees F with 90-95% humidity. Temperatures below this may cause pitting or softening of the tissue. Also be sure to not store peppers with ethylene-producing fruit. This will also shorten the shelf life and may cause discoloration of the peppers.

(my reply to a grower inquiry)

Red plastic mulches are supposed to block all light, like black plastic, so they should stop all weed growth. The purpose of the red pigment is to reflect wavelengths of light up to the plants that will promote growth. However, the red pigment in plastic mulch is very expensive, so manufacturers do not add any 'extra' for insurance and in fact some may skimp on it and that would allow light to pass thru, leading to weed growth. My feeling is that with red plastic you get what you pay for (in other words, avoid inexpensive brands).

Green plastic mulches are generally ‘selective wavelength mulches’ which are supposed to allow some PAR (photosynthetically active radiation) to get through, with the goal of  warming than plastics that block all light, but allowing less weed growth than clear plastic which allows all light to pass thru. There is variation in the amount of PAR that gets thru different brands of green mulches and also growing conditions will affect weed growth - i.e. if weeds germinate early but the crop canopy is slow to close that will lead to a lot of weed growth.

One company I know of does sell 'olive' colored selective wavelength mulch that supposedly blocks all PAR but somehow heats up the soil more than black plastic. The Ken-Bar web site is linked to my site (above) under ‘supplies and equipment’.

(my reply to a grower inquiry)

Thanks for making me review my plant physiology texts….it seems that flowering actually diverts photosynthate (sugary energy produced in the leaves) away from tubers, so lack of flowers and 'seed set' is probably a good thing in terms of yield, rather than anything to worry about. The reason you don't see flowers I think is because the very young flowers abort, probably due to environmental conditions and/or competition for photosynthate with other 'sinks' i.e. the tubers.

(adapted from David Handley, UMaine Extension)

Potato leafhopper injury is expected to be found soon in most locations – they recently already arrived here in Brattleboro, dropping down as storm fronts pass through. This insect commonly attacks alfalfa, beans and potatoes, although it also can damage melons, strawberries, raspberries and other crops. Symptoms include yellowing, stunting and curling leaves. On bean plantings that have had severe PLH feeding symptoms may look like virus infection. To scout for PLH, walk through your fields and lightly brush plants in different locations. When disturbed, the small, whitish adults can be seen flying off plants. Then look for small, light green leafhopper nymphs on the underside of the leaves. They are about 1/16th inch long and bullet-shaped. Don’t wait until you see clouds of adults flying around – start scouting for them now. The IPM threshold for PLH is quite low. Late maturing potato varieties including Katahdin, Elba, Green Mountain, Kennebec, and Blossom have some resistance, and likely will not need spraying. Yukon Gold, Red Norland, and most other varieties, are very susceptible. Check the New England Vegetable Management Guide for appropriate pesticides.