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Clemson HGIC, 'Tomato Diseases & Disorders', hgic.clemson.edu/factsheet/tomato-diseases-disorders (read 2026-07-11): 'tomatoes and related vegetables, such as potatoes, peppers, and eggplants, should not be planted on the same land more than once in three years.'

NC State Extension, 'Early Blight of Tomato', content.ces.ncsu.edu/early-blight-of-tomato (read 2026-07-11): 'Rotate every 2-3 years.'

NC State Extension, 'Septoria Leaf Spot of Tomato', content.ces.ncsu.edu/septoria-leaf-spot-of-tomato (read 2026-07-11): 'The fungus is known to overwinter on infected tomato debris and solanaceous weed hosts'; 'Rotate away from tomato for 2 or more years.'

NC State Extension, 'Verticillium Wilt of Tomato and Eggplant', content.ces.ncsu.edu/verticillium-wilt-of-tomato-and-eggplant (read 2026-07-11): 'The pathogen can survive in the soil without a host for up to 10 years in the form of structures called microsclerotia'; 'affects more than 300 host families.'

UMass Extension, 'Solanaceous, Verticillium Wilt', umass.edu/agriculture-food-environment/vegetable/fact-sheets/solanaceous-verticillium-wilt (read 2026-07-11): 'Verticillium wilt tends to affect a wide range of hosts and can infect many unrelated plants. Fusarium spp., on the other hand, have very narrow host ranges.'

UMass Extension, 'Potato, Scab', umass.edu/agriculture-food-environment/vegetable/fact-sheets/potato-scab (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'Common scab is controlled or greatly suppressed at soil pH levels of 5.2 or lower'; 'Avoid application of soil amendments that increase soil pH'; 'Increase rotation with nonhost crops to 3-4 years'; 'Maintain adequate soil moisture during and after tuber set.'

Wisconsin Horticulture Extension, 'Potato Scab', hort.extension.wisc.edu/articles/potato-scab (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'Scab tends not to develop when the soil pH is less than 5.2'; 'For home gardens, keeping the soil pH at approximately 6.5 is a good compromise'; 'Avoid using fertilizers (e.g., lime, calcium and potassium nitrate, or fresh manure) that may increase soil pH'; the disease 'can also affect other root vegetables such as beets, carrots, parsnip, radish, rutabaga, salsify and turnip.'

Clemson HGIC, 'Irish Potato & Sweetpotato Diseases', hgic.clemson.edu/factsheet/irish-sweet-potato-diseases (fetched in-container 2026-09-02; read by the maintainer 2026-09-02), common scab: 'Soil pH should be between 5.0 and 5.2'; 'avoid alkaline materials, especially lime and wood ashes, but also avoid mushroom compost and leaf compost. All of these will raise the soil pH'; 'Scab is favored under low soil moisture conditions, so the garden soil must be kept moist during the active growing period of the tubers (particularly 4 to 9 weeks after planting)'; 'Do not plant Irish potatoes after beets, carrots, radishes, parsnips, rutabagas, or turnips, which are also susceptible to scab.'

Univ. of Maryland Extension, 'Common Scab of Vegetables', extension.umd.edu/resource/common-scab-vegetables, and 'Growing Potatoes in a Home Garden', extension.umd.edu/resource/growing-potatoes-home-garden (both fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'The disease is encouraged by soil pH above 5.5'; 'Where potato scab disease is a persistent problem, a soil pH of 4.8 to 5.8 is recommended'; 'It can also be a problem of beets, carrots, parsnips, turnips, and radish.'

Univ. of Maine Cooperative Extension, Bulletin 2077, 'Potato Facts: Growing Potatoes in the Home Garden', extension.umaine.edu/publications/2077e (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'a soil pH of 5.3 to 6.0 is typical for potato production'; 'higher soil pH levels are more conducive to scab, a potato disease caused by a soil-borne pathogen'; 'Lime will also raise the pH of soil. A soil test will tell you how much lime to add.'

UMass Extension, 'Late Blight Management for Fall, Winter and Spring', umass.edu/agriculture-food-environment/home-lawn-garden/fact-sheets/late-blight-management-for-fall-winter-spring (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'Potato tubers that are infected with late blight and don't freeze or decay during the winter can carry the pathogen over the winter to next spring' - 'Left in the ground at harvest, down several inches in the soil'; 'Disposed of in a compost pile that does not fully decompose and does not freeze'; 'Tomatoes will not carry late blight over the winter, because freezing kills the whole plant'; 'Tomato seed, even from fruit that was infected with late blight, will not carry the pathogen'; 'In the spring (April - June), inspect last year's potato plot and any compost or cull piles for volunteer potato plants that might come up. If you find potato plants, pull them out and put them in the trash or destroy them.'

UMN Extension, 'Late blight of tomato and potato', extension.umn.edu/disease-management/late-blight (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'P. infestans can overwinter in Minnesota if protected in potato cull piles'; 'The host range is typically limited to potato and tomato, but hairy nightshade (Solanum physalifolium) is a closely related weed that can readily become infected'; 'Control volunteer potato plants, as infected plants can grow from infected tubers.'

University of Illinois Extension, 'Time Your Vegetable Plantings by Cold Hardiness' (2018-03-26), extension.illinois.edu/blogs/ilriverhort/2018-03-26-time-your-vegetable-plantings-cold-hardiness: very hardy vegetables 'withstand freezing temperatures and hard frosts without injury' (planted earliest in spring); frost-tolerant vegetables 'can withstand light frosts, but not freezing temperatures' (planted a couple of weeks later). LLM-summarised from a read of the page; it establishes the CATEGORIES but states no week offsets.

Colorado State Univ. Extension, GardenNotes #720, 'Vegetable Planting Guide', extension.colostate.edu/resource/vegetable-planting-guide (read 2026-07-25): hardy vegetables go in 'As early as 2-4 weeks before the date of the average last spring frost'; semi-hardy 'As early as 0-2 weeks before the date of the average last spring frost'; tender from seed 'around the date of the average last spring frost'; very tender 'Typically 2+ weeks after'. This is the source that states the offsets in weeks.

Where this rule sits in that range: hardy at 3 weeks is mid-range of CSU's 2-4. Half-hardy at 2 weeks is the EARLIEST EDGE of CSU's 0-2 - the most aggressive planting CSU sanctions, not the middle of it. The rule is inside published guidance at both ends, but it leans early on the half-hardy side and a gardener following it in a cold spring is taking that boundary.

Do not import an offset from another publication's hardiness scheme without checking the category names: CSU has no 'very hardy' tier, so a '4-6 weeks before frost' figure written against a four-tier scheme does not describe CSU's 'hardy'. Categories are not comparable across sources.

Penn State Extension, 'Potatoes in the Garden and the Kitchen', extension.psu.edu/potatoes-in-the-garden-and-the-kitchen (read 2026-08-21): 'Tubers exposed to sunlight (both before and after harvest) will turn green and should not be eaten'; 'Sun exposure causes the potato''s skin to produce chlorophyll and, along with it, two toxic compounds, solanine and chaconine. These cause the potato to have a bitter taste ...'; 'When the plant stems are 8 to 12 inches tall, gently hill or mound the soil around the base of the plant, covering most of the stem. You may repeat this mounding practice two or three times as the plants grow.'

UMN Extension, 'Growing potatoes in home gardens', extension.umn.edu/vegetables/growing-potatoes (read 2026-08-21): 'Green skin occurs when potato tubers experience exposure to light'; 'Hilling also keeps any shallow tubers from exposure to light and turning green.'

UMN Extension, 'Growing potatoes in home gardens', extension.umn.edu/vegetables/growing-potatoes (read 2026-08-22): 'Buy disease-free seed tubers from a certified grower or seed distributor'; 'Every piece must have at least one "eye," an indentation that will produce the new shoot'; 'Place the cut pieces on a tray at room temperature (between 60 F and 70 F) for a few days until the cut surface has dried out and formed a tough protective layer'; 'Do not plant potatoes purchased at the grocery store, as the store may treat them with chemicals to keep tubers dormant, in which case they will be slow to grow. Diseases may also infect the potatoes, which can remain in the soil for a long time.'

Utah State Univ. Extension, 'How to Grow Potatoes in Your Garden', extension.usu.edu/yardandgarden/research/potatoes-in-the-garden (read 2026-08-22): 'When buying seed tubers, ask for certified seed as this will help reduce the potential for introducing disease into the garden'; 'When cutting, make sure the seed piece weighs at least 2 ounces and has one or more "eyes"'; 'saving your own seed potatoes leads to a buildup of viruses and diseases that eventually will cause serious problems in the garden. Whenever possible, purchase and plant certified seed to help control many of the problem diseases potatoes experience.'

UMN Extension, 'Early blight in tomato and potato', extension.umn.edu/disease-management/early-blight-tomato-and-potato (fetched in-container 2026-09-25, read 2026-09-25): 'Larger spots have target-like concentric rings.'; 'The early blight pathogens both overwinter in infected plant debris and soil in Minnesota.'; 'Lower leaves become infected when they come into contact with contaminated soil, either through direct contact or when raindrops splash soil onto the leaves.'; 'Cover the soil under the plants with mulch, such as fabric, straw, plastic mulch, or dried leaves.'; 'Water at the base of each plant, using drip irrigation, a soaker hose, or careful hand watering.'; 'Do not remove more than one-third of the plant's leaves.'; 'Rotate out of tomatoes and related crops for at least two years.'; 'Spores (reproductive structures) can germinate between 47° and 90° F and need free water or relative humidity of 90% or greater.'; 'Fruit spots are leathery and black, with raised concentric ridges.'; 'Staking will also reduce contact between the leaves and spore-contaminated soil.'; 'Remove leaves with leaf spots and bury or burn them.'; 'There are many resistant tomato cultivars available, often designated with an "EB" in seed catalogs.'; 'In the fall, remove or bury infected plants to reduce the likelihood of the pathogen surviving into the following year.'