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 MOFGA's 2009 Pest Reports - Compiled by Eric Sideman, PhD Minimize

Late Blight Recap | September 3 | August 12 | July 22 | July 13 | June 25 | June 19 | June 14 | June 1 | May 19
 
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Pest Report - September 3, 2009

LATE BLIGHT: It is late in the season and the calls I receive now are not about saving the crop but about avoiding the problem next year. At last I have good news. The late blight fungus does not live through the winter in the northeast in the soil or on crop debris, UNLESS the crop debris does not die. The spores of late blight are not very resistant to the environment and remain viable only a short time. In other words, late blight survives in the Northeast essentially only in potato tubers. So, with that bit of information you can figure out what to do. Here are some answers to questions you still may have.
  • Freezing kills tomato and potato plant tissue so late in the season, when you are no longer worried about the disease spreading, the best practice may be to leave the plant debris on the surface of the ground to freeze.
  • Buried potatoes may not freeze and are the most common means by which the disease survives the winter. Next year scout for volunteer potato sprouts and kill them as soon as you see them so they are not a source of spores to start the problem again.
  • Use only disease free potato seed.
  • Late blight does not survive in tomato seed. Be sure to ferment well if saving your own seed.
  • Composting will kill the late blight fungus and spores, BUT it has to be good and hot composting. AND, poor composting is worse than just leaving the debris on the ground. If the compost pile, or any portion of the compost pile gets warm but not hot then you may have diseased tissue not freezing during the winter and the pathogen surviving.
  • Cure potatoes for a few weeks after harvest before putting into winter storage in hopes of identifying any that have late blight. Blighted potatoes are likely to rot in storage.
SQUASH: HARVEST PERIOD, STORAGE, AND VARIETY SELECTION TO OPTIMIZE EATING QUALITY IN SQUASH (Reprinted from the Umass Vegetable Notes). The record rainfall in June & July, followed by the recent heat, has led to a boom in fruit rot caused by Phytophthora capsici. Given the wet conditions and high disease pressure we’ve experienced this year, it makes sense to get your winter squash and pumpkins out of the field as soon as possible to help reduce the risk of fruit rots. It can be difficult, however, to assess whether or not the fruit is truly ready for harvest – especially in dark green squash, such as acorn varieties. This article aims to provide some insight into judging the ideal time to harvest squash and pumpkins. There are three major species of squash that are grown worldwide – Cucurbita pepo, C. maxima, and C. moschata. The species C. moschata includes calabaza or tropical squash, round to oval pumpkins grown in the Midwest for pie processing, and the popular butternut varieties, highly regarded for excellent shelf life. The species C. maxima includes the large show pumpkins, Golden Delicious type processing squash, Hubbard varieties, and buttercup/kabocha varieties, the latter esteemed for their exceptional eating quality. Lastly, C. pepo is the species having the greatest variation in type, including hard-shelled gourds, summer squash, ornamental pumpkins, and squash. In North America, acorn is the most popular C. pepo squash, but striped Delicata and Sweet Dumpling varieties are known for having good eating quality. The demand for acorn squash has been adversely affected by generally poor quality of popular commercial varieties and the practice of harvesting squash before it reaches maturity. Components of eating quality: People differ in their preference for flavor components and degree of moisture in squash. Nonetheless, connoisseurs of squash usually prefer a relatively dry squash that has a pasty, slightly moist texture after cooking and a high level of sweetness. High sugars not only contribute to a desirable sweet taste, but also mask undesirable flavor components associated with certain varieties. Sugar levels can be estimated easily by pressing juice from a small sample of flesh and measuring soluble solids in the juice with a hand-held refractometer. Relative sugar content is given in units of percent soluble solids (or oBrix). Soluble solids levels of 10% are passable, but generally levels of 11% or greater are considered necessary for good eating quality in squash. The pasty texture of squash is attributable to starch. At harvest starch comprises about two thirds of the dry matter of squash, so squash with high dry matter also have high starch content. Starch provides substrate for conversion to sugars during the latter stages of squash maturation and during subsequent storage. Squash with low dry matter, generally less than 16%, lack sufficient starch levels to produce the combination of pasty texture and degree of sweetness desired for acceptable eating quality. In varieties with low dry matter, starch is rapidly depleted during storage by conversion to sugars, and the texture of the squash becomes watery and fibrous.

Stages of squash development: To understand how harvest period, storage and variety selection can affect eating quality, it is necessary to understand basics of squash development and maturation. This process includes not only the development of flesh quality, but also the effect of seed development on maintaining flesh quality. Small-fruited varieties of squash, such as acorn, reach close to full size within 15 to 20 days after pollination (DAP) and subsequent fruit set. Dry matter and starch accumulation begins shortly after fruit set, but is most rapid between 10 and 20 DAP and reaches a maximum at 30 DAP. Sugar levels, on the other hand, are very low at 25 DAP, but continue to increase until maturation of squash at about 55 DAP (Figure 2). Some varieties, however, lack adequate sugar levels even at mature harvest, and need to be stored to develop sugar levels suitable for good eating quality. Even though the dry matter of the flesh (mesocarp tissue) peaks at about 30 days after pollination, seed development takes much longer. If a squash is cut open at 20 DAP, the seeds appear to be full size. This is because the seed coat, the leathery covering over the embryo, reaches full size by this time. But if the seed is cut in half, the embryo is actually barely visible at this time, being about an eighth to a quarter of an inch in length. The embryo expands rapidly and largely fills the seed coat cavity by 35 days after pollination. However, dry seed biomass (seed fill) continues almost linearly until about 55 DAP. Thus, a squash fruit can be considered to reach full maturation when seed development is complete at about 55 days after pollination. If fruit are picked immature, seed development continues in stored fruit at about the same rate as in fruit left on the plant. Seed development in an immature, detached fruit occurs at the expense of depletion of nutrient reserves in the fleshy tissue, thereby reducing dry matter (mostly starch) and lowering eating quality. Post maturation changes occur in stored fruit. There is a progressive moisture loss during storage, so fruit fresh weight decreases. Respiration consumes carbon in the form of sugars, and starch continues to degrade to replace the sugar consumed by respiration. The eating quality of squash varieties with low sugar at harvest will initially be enhanced in storage because sugar levels increase. Eventually, however, long storage time will deplete starch levels to a point where the texture of the squash is compromised. To maximize shelf life, squash should be stored at 55 to 60° F with moderately high relative humidity (50 to 70%). Because seed maturation is not complete until 7 to 8 weeks after fruit set, it is important to maintain a healthy plant until at least 50 days after fruit set. This insures a continuous supply of photosynthates (carbon source produced from photosynthesis) to the developing fruit. Seeds are the primary sink for assimilates such as sugars, so if photosynthesis is impaired by disease or insect feeding, nutrients for the developing seed are withdrawn from the flesh, depleting starch levels and lowering eating quality. Harvest period and eating quality: Because fruit and seed development are similar in all three species of squash, their recommended harvest periods are similar. Butternut squash do not reach their characteristic tan color until late in development, so premature harvest before starch accumulation and seed fill are complete is generally not a problem. With kabocha varieties, it is actually desirable to harvest them before complete seed maturation, about 40 to 45 days after fruit set when the fruit are still bright green. New Zealand studies indicate that rind hardness is maximum around 40 DAP, so fruit harvested at 40 days suffer less damage to the fruit surface, and in turn, less chance for disease entry during subsequent storage, than fruit picked during later stages. Kabocha squash are also susceptible to sunburn damage and changes in rind color to brownish green, so it is best to harvest the squash before fruit are exposed to direct sun as the vines die down. Kabocha squash have a high dry matter content, usually 20 to 30%, and a small seed cavity, so that any seed maturation following harvest has a minimal effect on depleting starch reserves in the flesh. Acorn squash present the most difficult problem with respect to determining harvest time. Most modern acorn varieties not only reach near full size within two weeks after fruit set, but also develop a dark green to black mature color. For this reason, acorn squash harvested for the large wholesale markets are often picked immature. This can be easily observed in supermarkets by noting that the rind on the ground side of the squash is light green or light yellow rather than dark orange coloration of mature fruit. If these immature squash are sampled, they are found to have very low sugar levels. If such immature squash are left in storage, sugar content will increase, but the starch will be depleted both by respiration and movement of nutrients from the flesh to the developing seed, and this results in poor eating quality. The problem of poor quality in prematurely harvested squash is further exacerbated because most commercial acorn varieties and many of the newer striped varieties have inherently (genetically determined) low dry matter and starch levels. How do you determine when to harvest? Most acorn varieties are semi-bush and set most of the crown fruit within about a week period. Modern hybrids tend to produce some female flowers before male flowers appear and these usually abort unless there are other varieties of C. pepo nearby supplying pollen. But this is shortly followed by a period of both male and female flowering and fruit set. Some later fruit sets will occur on runners, but these fruit are usually undersized and lack quality, and so should not be harvested and sold. These late set fruit are a drain on photosynthates, and pruning these fruit off of the plant can actually increase quality of the crown set fruit. By noting the initial flush of male and female flowers on a semi-bush squash cultivar, a grower can estimate the approximate time when fruit set occurred, and delay harvest until about 50 days or more from the fruit set period. Another approach is to check the ground spot on the fruit, and not harvest fruit until the spot turns orange. Some of the newer striped varieties of C. pepo will show some color changes with maturation, but the color change, say from white to tan between the stripes or stripes changing from green to orange, may occur well after the fruit are ripe enough to harvest. So with these, I think that it is better to keep track of the approximate date of fruit set. However, if you observe a color change that correlates with maturity in a particular variety, then you can use that as a harvest indicator. How about variety selection? That is a tough call. I have found that most modern hybrids being commercially sold lack the eating quality of a good Sweet Dumpling or Delicata squash. UNH has developed some high quality acorn and sweet dumpling type varieties that are being released to the seed industry. High Mowing Organic Seeds offers a UNH-developed, sweet dumpling hybrid, Sugar Dumpling, which also has intermediate resistance to powdery mildew. Johnny’s Selected Seeds is in the process of producing one of my PMR mini-acorns, and currently sells an acorn hybrid, Tip Top, that has good eating quality. Cornell Bush Delicata is another variety in this class with good eating quality and powdery mildew resistance. There are several other varieties available that have reasonably good eating quality, so growers will just have to evaluate them to determine if they fit into their particular farm and marketing situation. - Brent Loy, Department of Plant Biology University of New Hampshire.

IMPORTED CABBAGE WORM. The white butterflies are flying all around the cabbage family plants now and laying eggs, and the larvae (green caterpillars) are chewing holes in the leaves. If you have heading broccoli you must know your customers because some folks are really turned off by the critters floating to the top of the water in the cooking pot. You may want to spray that even though the crop is beyond risk.

This pest overwinters as a pupa and there are 3-4 generations per year. This means that once you start seeing the butterfly you should start scouting for the caterpillar in about a week. Bt (Dipel 2X or Dipel DF) or Entrust work very well in controlling the caterpillar. None of these materials lasts in the field and so should only be sprayed when the caterpillars are there in large enough numbers to warrant it.

Destroy or bury crop residue after harvest so as not to allow the caterpillars to continue to feed and complete their life history and thus a larger second generation.

ALTERNARIA LEAF BLIGHT OF CARROT: The lesions on the leaves start along the margin as spots but they grow together and the leaflets shrivel and die. This disease effects the older leaves much more than the younger leaves and so it is common to see healthy young leaves growing through the mass of dead old leaves. The pathogen survives on seed so be sure to buy seed from a good source. It overwinters on crop residue and on related weeds. It is spread by wind and water, mostly happening when the leaves are wet long periods. A two to three year rotation and good sanitation are important. For late season carrots that have healthy young foliage a boost of nitrogen now may help carrots to size up before harvest.

Late Blight Recap | Page 2 of 10 | August 12

    

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