Vegetable Notes 2024 Vol. 36:17

Basil

Downy mildew was observed in Dukes Co. last week on an unknown variety of basil. Basil downy mildew overwinters in southern states where basil is grown year round and blows northward on storm fronts throughout the summer. It can also be carried northward on infected transplants. Growing resistant varieties starting in July is the best management practice. Based on varietal responses in UMass trials, a recommended combination is to grow Prospera and Passion, and then introduce the new Prospera Active (which contains a 2nd BDM resistance gene in addition to the gene in the original Prospera) later in the season. Conventional growers can increase their protection with fungicides. Effective materials include Quadris, Reason, Revus, Orondis Gold, Orondis Opti, Ranman, and phosphorous acid fungicides. For more information on BDM-resistant basil varieties and labeled fungicides, see our article in the December 14, 2023 issue. Let us know (umassveg@umass.edu) if you see downy mildew on any of the resistant varieties this summer so that we can help track the evolution of this important pest.

Cucurbit downy mildew has been confirmed in southeastern NY, and with the recent northeastward storms, it’s likely that CDM spores are now in MA. All nearby CDM reports are on cucumber, which means that currently, cucumber and cantaloupe are most at risk for disease development. It is now recommended that MA growers add targeted fungicides to cucumber and cantaloupe spray programs. Rotate between at least 2 classes of targeted fungicides. Growers should also be regularly applying broad-spectrum protectant fungicides. Effective targeted CDM materials include Elumin, Zing, or Gavel (FRAC Group 22), Orondis Opti (49 + M05 – also effective against powdery mildew), Zampro (40 +45), Omega (29), Previcur Flex (28), Curzate or Tanos (27), and Ranman (21). Materials that are not recommended due to resistance development are Presidio, Revus, and Forum. Broad-spectrum protectant materials for CDM are chlorothalonil and copper, both of which are also effective against powdery mildew. Mancozeb is effective against CDM but not powdery mildew, so should only be used as a protectant if a crop has no powdery mildew present. Please let us know if you suspect CDM in your crop so that we can confirm the presence of this important disease – umassveg@umass.edu or 413-577-3976.

Squash lady beetle was identified last week in Dukes Co. This pest looks very similar to Mexican bean beetle (MBB)—adults look like bronze lady beetles, and larvae are yellow and spiny. MBB will feed only on legumes, while squash lady beetles feed on cucurbit crops. We have never seen this pest before but it was widespread on several farms on Martha’s Vineyard, and is reported to be more common along the coast. At small scales, hand picking these beetles off of a crop is effective. Insecticides effective against striped cucumber beetle are likely effective against squash lady beetle.

Phytophthora blight is continuing to develop, both in cucurbit and pepper crops. Fruit rots caused by Phytophthora blight will start on the side of the fruit that is in contact with the soil. The lesion will develop a wet, powdery sporulation that looks similar to powdered sugar. This is in comparison to Pythium rot, which is a much less aggressive disease but will also cause fruit rot in cucurbits. Pythium forms fluffy white mycelium (fungal growth) on infected fruit.  

Squash vine borer numbers are continuing to drop and most trapping sites are below threshold for spraying in all cucurbit crops. There may be a second generation of adults that emerge in mid-August, and we will continue to monitor for that. The larvae from that generation often bore into winter squash fruit.

Pepper maggot flies are continuing to lay eggs in pepper fields. Growers are often not aware of pepper maggot infestations because infested peppers become rotten and are culled in the field during harvest or in the pack house. Cut open rotten peppers or peppers that fill with water during washing and examine for 1 cm-long maggots feeding inside. For information on management, see last week’s article.

Where might you see ECB? Before there is silk, caterpillars bore into the tassel or stalk. The weakened stalk will often flop over and you will see flagging tops in the field. Once there is silk in the field, ECB will enter the ears through the silk channel, or bore directly into the side of the ear. You may often see them in ripening corn boring into the side of the ear next to the corn stalk.

Where would you be seeing CEW now? CEW are in ear tips—look for messy frass and/or chewed up silks.

Fall Armyworm

Fall armyworm does not overwinter in the Northeast; moths are blown northward on storm fronts, usually starting in mid-July. CEW is also blown in on storms, though they don’t always move together. Females prefer laying their eggs in whorl-stage corn. Eggs are laid on leaves and hatch in ~5 days. The larvae feed in the whorl and newly forming tassel, creating large, ragged holes in the leaves and dropping big clumps of frass. Larvae feed for 15 to 20 days.

Adults are ¾-inch long, mottled dark gray moths, with some light spots on their wings and an obvious white area at the extreme tips of their wings. 

Eggs are laid in masses on leaves and are surrounded by fuzzy hairs from the female moth.

Caterpillars are light-tan to dark black, with some longitudinal striping along their bodies. Their bodies are smooth, unlike the hairy CEW caterpillars. When viewed head on, FAW head capsules are divided by an inverted “Y”, compared to the solid CEW head capsule.

Early blight (Fungal: Alternaria solani) usually affects potato foliage but tuber infections can also occur. Tuber lesions are dark, sunken, and circular, and are often bordered by raised, purple to gray tissue. The underlying flesh is dry, leathery, and brown. Lesions can increase in size during storage, causing tubers to become shriveled.

Fusarium dry rot (Fungal: Fusarium spp.) causes internal, light to dark brown or black dry rot of the potato tuber. The rot may develop at an injury site, such as a bruise or cut. The pathogen penetrates the tuber, often rotting out the center. Extensive rotting causes the tissue to shrink and collapse, usually leaving a dark sunken area on the outside of the tuber and internal cavities.

Silver scurf (Fungal: Helminthosporium solani) affects only tuber periderm (skin). Lesions start at the stolon end of the tuber as small, pale brown spots which may be difficult to detect at harvest but will continue to develop in storage. In storage, lesions may darken and the skin may slough off. Many small circular lesions may coalesce to form large affected areas. Tubers may also dry out and become wrinkled due to excessive moisture loss in storage.

Black dot (Fungal: Colletotrichum coccodes): On potato foliage, symptoms of black dot are nearly indistinguishable from early blight. On tubers, it produces large discolored areas that can easily be mistaken for silver scurf. Under a 10X lens, tiny black sclerotia are visible on the surface of the affected tissue. 

Black scurf and Rhizoctonia canker (Fungal: Rhizoctonia solani): Black scurf is purely cosmetic and does not reduce yield, even in storage. Irregular, hard, black masses that develop on tuber surfaces are overwintering structures (sclerotia) of the fungus. Development of these sclerotia may be minimized by harvesting tubers soon after vine-kill and skin set. While the sclerotia themselves do not cause damage, they allow the pathogen to survive in the soil and serve as evidence of its presence. In cool, wet soils, R. solani can cause dark, sunken lesions on underground sprouts and stolons. These lesions can cut off the supply of nutrients and kill tubers, or can reduce the transfer of starches to the tubers, reducing their size. Cankers can also form on the tubers themselves, usually at the stolon or in lenticels. Tuber cankers vary greatly in size, from small and superficial to large, sunken, and necrotic.

Pink rot (Oomycete: Phytophthora erythroseptica): Pink rot infections start at the stolon end of tubers and result in rotten and discolored periderm with a clear delineation between healthy and diseased tissue. When exposed to air, tuber flesh turns pink and then brown-black. 

Pythium leak (Oomycete: Pythium spp.): The Pythium species that cause leak infections invade tubers through harvest wounds and continue to develop in transit and storage. Infections result in internal watery, gray or brown rot with well-defined red-brown lines delineating healthy and diseased tissue.

Late blight (Oomycete: Phytophthora infestans) affects potato foliage and tubers. Foliar symptoms start with brown to black, water-soaked lesions on leaves and stems, which produce visible white sporulation at the lesion margins under humid conditions. Whole plants and fields may collapse rapidly. Tuber infection is initiated by sporangia from foliage being washed down into the soil and usually begins in wounds, eyes, or lenticels. Lesions are copper brown, red or purplish and white sporulation may occur on tuber surfaces in storage or cull piles. Infected tubers are susceptible to infection by soft rot bacteria, which can turn entire bins of potatoes in storage into a smelly, rotten mass. (Late blight has not been seen in MA since 2017; there is currently a report in central NY and a few reports in southern Ontario.)

Potato virus Y can cause necrotic ringspots on tubers, depending on which strain of the virus is present, which potato variety is grown, and the time of infection. Affected tubers have roughened rings of darker brown or reddened skin. Necrosis beneath the rings may extend into the tuber flesh. Necrotic symptoms in tubers often increase after storage. Potato varieties vary in their susceptibility to PVY and the symptoms they exhibit on foliage and on tubers; Yukon Gold is particularly susceptible to tuber necrosis. Management of this disease starts with sourcing certified disease-free seed tubers, then preventing spread by aphids by planting buffers around the potato crop.

Physiological Disorders

Black heart is caused by lack of oxygen during storage, which causes the tissue to die from the inside out and turn black. The condition is not reversible, but if you notice it quickly and correct your storage conditions you can prevent the whole crop from being affected.

Brown center and hollow heart are internal physiological disorders of potato that often occur together. Brown center is an area of dead pith cells that turn brown, while hollow heart is a star- or lens-shaped hollow area in the center of the tuber. These disorders make fresh-market tubers unattractive and can reduce repeat sales. Severe hollow heart negatively impacts the quality of chip-processing potatoes and can result in shipments not making grade. Both disorders are related to stress, and occur at a higher incidence when growing conditions abruptly change during the season. Brown center and hollow heart likely form during tuber initiation but could also form during tuber bulking. If the disorder occurs during the early part of the season, it most often begins as brown center that forms in the stem-end of the tuber, while late-forming hollow heart usually occurs near the bud-end with no brown center symptoms. Conditions such as soil temperatures below 56°F for 5-8 days, or available soil moisture above 80% initiate brown center formation. Incidence of brown center and hollow heart also increases with periods of stress caused by high or low soil moisture, especially if heavy rains occur suddenly after a dry spell. Large tubers are more prone to develop the disorder, so using closer spacing and avoiding skips in the row can reduce incidence of brown center and hollow heart. There are also differences in the susceptibility of potato varieties to both of these disorders.

--Written by Susan B. Scheufele

Blossom End Rot

Blossom end rot is characterized by dark brown or black sunken areas at the blossom end of the fruit. The lesions are a direct result of a localized calcium deficiency at the blossom end. The dead tissue is initially brown or white and leathery but often becomes colonized by the saprophytic fungus Alternaria alternata, which only infects dead tissue and produces fuzzy black sporulation. Although calcium deficiencies in the soil are sometimes responsible for blossom end rot, much of the time, blossom end rot is the result of plant water stress. When temperatures increase, so does water demand, and developing fruits have high calcium demand. Calcium is taken up primarily through the water-conducting tissues of the plant, so when the soil is dry, or overly saturated, or the plant’s roots are compromised and the plant isn’t taking up water, calcium deficiencies develop. The blossom end of the fruit is the last to accumulate calcium, so this is where deficiencies appear. Signs of blossom end rot typically appear two weeks after the deficiency in the plant has occurred. 

Blossom end rot can be prevented by regular watering to avoid extreme fluctuations in soil moisture. In high tunnel tomatoes, it is recommended to use more than one drip line per bed to ensure that the entire extensive root system is able to supply sufficient water to the lush foliage. Be careful to avoid overwatering, though, as this can lead to root rots or other physiological disorders. Use of soil moisture monitors can help dial in watering needs. High soluble salts, low calcium, and high cation (potassium, magnesium or ammonium) levels in the soil may also contribute to blossom end rot; other cations may out-compete calcium on soil exchange sites, making it unavailable to the plant.

Leaf roll is usually a reaction by the plant to conserve water by reducing the surface area exposed to solar radiation. This disorder is often seen just after plants are heavily pruned under dry soil conditions, but oddly enough, leaf roll disorder also has been found to be caused by excess soil moisture coupled with extended high temperatures. If the tomato plant’s foliar growth is more vigorous than root growth and we are hit with a hot, dry period, the foliage may transpire water faster than the root system can absorb it from the soil, and the plant’s reaction is to roll it’s leaves up to reduce transpiration. This disorder primarily affects lower leaves. Leaf rolling can also result from growing high-yielding cultivars under high-nitrogen fertility programs. Cultivars selected for high yield or early ripening tend to be the most susceptible and indeterminate cultivars are more sensitive than determinate. The good news is that leaf roll rarely affects plant growth, fruit yield, or fruit quality. Some viruses can look similar to tomato leaf roll, but if the symptoms appear suddenly, involve many of the plants in a field, and largely affects lower leaves, it is probably just physiological leaf roll.

Management: Reduce symptoms by maintaining consistent, adequate soil moisture (~1 inch per week during the growing season, which will also help with calcium uptake, reducing blossom end rot incidence). Do not prune heavily during hot, dry conditions or over-fertilize with nitrogen.

Catfacing

Tomatoes are considered “catfaced” if the blossom scar is enlarged or perforated. This can happen to both field and greenhouse tomatoes and is more common in heirloom cultivars and varieties with rounder and larger fruit. Often, the fruit becomes extremely misshapen, but fruit distortion is not necessary to classify it as catfaced. This disorder has not been extensively researched and is still not fully understood. Cold temperatures during flowering have been shown to increase incidence of catfacing, as have extreme fluctuations in night versus day temperatures. Damage from thrips to the side of the pistil of tomato flowers can also cause this disorder, and under some conditions, pruning and high nitrogen levels can increase catfacing incidence. Catfacing can increase the chances of fruit becoming infected via the rough blossom scar by black mold rot, a disease caused by several different fungi.

Management: Avoid excessive pruning and nitrogen fertilization. Avoid low greenhouse temperatures for both greenhouse tomatoes and transplants. Use cultivars that are less prone to catfacing. Remove catfaced fruit so they don’t drain plant nutrients. 

Stitching/Zippering

Stitching or zippering are terms for when a thin, brown, necrotic scar extends from the stem to the blossom end on fruit. The longitudinal scar has small transverse scars along it, making it resemble a zipper or seam. Fruit can have one or several scars. This disorder is purely cosmetic and does not affect the edibility of the fruit but may render fruit unmarketable. Zippering is caused by anthers (the pollen-producing flower part) fused to the ovary wall of newly forming fruit. This disorder occurs more frequently in cool weather.

Management: Plant cultivars that are less susceptible to stitching/zippering. Avoid low greenhouse temperatures.

Fruit Cracking

Fruit cracking can occur in several different patterns – radial cracks starting at the stem scar, concentric cracks circling the fruit shoulders, small cracks around the shoulders called “rain checking”. These cracks are caused by excessive water uptake by the plant during fruit development, when interior tissues expand faster than the inflexible skin. High temperatures can make fruits more susceptible to cracking, and cultivars with large fruit tend to be more susceptible. Opportunistic fungi and bacteria can enter the fruit through cracks and cause fruit rots.

Yellow Shoulder

Yellow shoulder is commonly caused by potassium (K) deficiency in tomatoes. Recent surveying by several New England state Extensions has shown that tomatoes, especially high tunnel tomatoes, have very high potassium requirements and crops tend to be deficient. Current nutrient recommendations advise New England growers to fertilize based on the potential yield of your crop (with indeterminate, disease-resistant cultivars planted early into heated tunnels expected to yield more than heirloom or determinate cultivars planted later). See the full recommendations in the high tunnel tomato section of the New England Vegetable Management Guide. One recommendation for K fertility in high tunnel tomatoes is to apply ¼ lb of K/1000 ft2 through the drip every week for 10 weeks, using potassium sulfate fines. In both the field and high tunnels, the following factors can also lead to K deficiency in tomatoes: waterlogged and/or compacted soils, above-optimal nitrogen application rates, excessive application of potassium competitors, and excessively large or dense canopies. Some cultivars, including those with green shoulders that lack the uniform ripening gene, are also more prone to developing yellow shoulder than others. High temperatures and sun can also cause this disorder, both by directly damaging fruit tissue and by inducing water stress that can limit the uptake of K.

Blotchy ripening, gray wall, and internal whitening

These three disorders are not well-understood and tend to be confused with each other. Blotchy ripening is characterized by yellow or orange discolored areas on tomato fruit surfaces. Tomatoes with gray wall have grayish-brown discolorations on the fruit wall and may also exhibit internal browning. Gray wall typically appears on green fruit before ripening. "Gray wall" is the term used when the outer fruit walls turn brown or gray and collapse, compared to "internal whitening", which refers to when the outer and inner fruit walls become white and corky. Factors that increase the severity of these disorders include cloudy weather, wet and cool conditions, high nitrogen, low potassium, and compacted soils. Good irrigation and nutrient management will reduce the risk of these disorders. 

Ripening disorders, including yellow shoulder, blotchy ripening, gray wall, and internal whitening, are most prevalent when air temperatures during mid- to late stages of fruit ripening are extreme (e.g., below 60°F and/or above 90°F) or highly variable, when humidity levels remain high, and/or when these conditions prevail and light levels are low. Unfortunately, these are not uncommon conditions in New England and are largely out of our control. Tomato mosaic virus can cause similar symptoms of uneven fruit ripening and should be ruled out as the underlying cause.

Planting tomatoes onto white plastic mulch and using shade cloth can help keep crops cooler and increase marketable yields. Research has shown that a 30% filtering shade cloth increases marketable yields by 20-50%, depending on the year. A 4 ft-wide shade cloth covering even a quarter of the crop canopy is sufficient to achieve this increase in yield, and the cloths may be used for many years.

--UMass Extension Vegetable Program