Late-season foliage loss in winter squash

Wednesday, September 11, 2019

Hurricane Dorian caused widespread damage in Nova Scotia.  Many cucurbit plants lost significant amounts of foliage.  Late-season foliage loss in winter squash and pumpkin leave the fruit exposed to sunscald.  If your crop has lost enough foliage so that there is no longer sufficient canopy protection to prevent sunscald, it is recommended that you harvest your squash and pumpkins.

Fig. 1. Sunscald on butternut squash, three
weeks post-storm.
Sunscald injury (Figure 1) occurs when cucurbit fruit are exposed to direct sunlight during the later stages of fruit ripening - while this year this is predominantly due to Hurricane Dorian, this can also occur as a result of powdery mildew, bacterial wilt, Phytophthora blight, or other pests.  Sunscald injury will result in rind tissue collapsing in the exposed area.  Initially the rind may appear water-soaked - on pumpkins this can be hard to observe at first and the area might just appear slightly sunken.  Darker skinned squash (particularly green acorn) will show sunburn fairly quickly.  Affected tissues can look bleached (Figure 2) and the rind may slough off when rubbed.  Over time, the fruit tissue may become tan, brown, or black (Figure 3), a result of secondary pathogens invading, and will eventually collapse.

Fig. 2. Sunscald on Sweet Mama, three weeks post-storm.
Typically, it is recommended that pumpkins and winter squash are harvested when they are fully mature for optimum quality and storability.  Fully mature pumpkins and winter squash will typically have a more hardened skin that is resistant to puncture with a thumbnail.  Fully mature
fruits will often have lost rind surface gloss and may have ground-spot yellowing.  Mature fruit that are left attached to vines are prone to infection in the stems and fruit.  As long as pumpkins have started to turn colour, they will continue to ripen off the vine if held under proper conditions.  When harvesting, avoid bruising the fruit or damaging the skin as this will result in storage rots setting in.

Proper curing and storing will greatly enhance the storability of your crop.  Note that chilling injury will occur if stored below 10°C (50°F).

Fig. 3. Sunscald on Sweet Mama with secondary infection,
three weeks post-storm.
The best conditions for ripening pumpkins are in a well-ventilated area at 27- 30°C (80-85°F) with relative humidity of 80-85%.  The curing period should be about 10 days.  This will help the fruit to ripen, the skin to harden, and any wounds to heal.  After curing, pumpkins should be stored in a cool, dry place (10 - 15°C or 50 - 60°F) with a relative humidity of 50-70%.

The best conditions for ripening squash are in a well-ventilated area at 21- 27°C (70-80°F).  How long squash will typically keep under optimal storage conditions depends on squash type.  Cornell has a chart outlining optimal storage temperatures and relative humidity based on squash type, which can be found here.  Note that if Acorn squash is stored below 13°C, it will become stringy.

The goal is to maintain enough humidity so that the fruit doesn't desiccate but not so much that condensation occurs which can promote bacterial and fungal growth.  Storing at higher temperatures results in weight loss, and at lower temperatures chilling injury can occur.  All fruit that is placed in storage should be free from disease and in good condition to promote the longevity of the crop's storage life.  Pressure bruises from packing fruit too tightly or too high can also result in reduce marketability of your squash and pumpkins.