Harvest Rots and Disorders of Pumpkins and Squash

Fall weather conditions can make or break a pumpkin crop.  Harvest rots include pythium (figure 1), black rot (figures 2 and 3), phytophthora and white mould. In a dry year, losses are quite low.  However in a wet year, growers may loose a significant amount of crop to these pathogens.

Minimize losses by harvesting pumpkins and squash as soon as they are mature.  If powdery mildew is present in the field, it is especially important to cut the fruit from the vine in order to prevent the spread of the disease into the handle.  The longer the crop stays in the field, the more risk of rot pathogens becoming established.  Pumpkins with >60% orange colour will continue to ripen after they are removed from the vine.

Maintain storage temperatures between 10-15 C and a relative humidity of 50%.  Ensure good airflow through the storage area.  If indoor storage is not an option, pile pumpkins on a clean dry surface.  A concrete pad or wooden pallets will do the job quite nicely.  At bare minimum, creating windrows in the field, 2-3 ft deep, can help to improve field recovery under wet weather conditions.  Avoid creating deep piles of fruit.  Hot spots may occur at the centre of a deep pile, increasing the likelihood of rot.

Other common diseases affecting the fruit include scab (figure 4), septoria (figure 5) and oedema (figure 6).  Scab, septoria and black rot (alligator skin) are also foliar diseases.  Infections on the fruit usually occur during fruit sizing, while the rind is still very tender and susceptible to infection.  Foliar fungicide applications at this time can help to reduce the incidence of scab, septoria and black rot at harvest. Occasionally these diseases will infect the fruit with no apparent signs of leaf infections.  Careful monitoring throughout the season can help to identify epidemics early on.

Pythium Rot on Mini Pumpkin
Figure 1. Pythium Rot on Mini Pumpkin

Pythium is common to most agricultural soils and infects a wide range of crops. Initial infections often occur where the fruit rests on the soil, or at the junction of the stem and the fruit, where water tends to collect.  Prevent development during storage and shipping by ensuring that the fruit are completely dry before packing.

Black Rot (Alligator Skin) on Butternut Squash
Figure 2. Black Rot (Alligator Skin) on Butternut Squash
Black Rot Pumpkin
Figure 3. Black Rot Pumpkin

Black rot (alligator skin) is caused by the gummy stem blight pathogen.  It survives in crop residue and in infected seed.  Follow a 3-4 year rotation away from all cucurbit crops and buy only certified, disease-free seed.

Scab of Pumpkin
Figure 4. Scab of Pumpkin

Scab survives in the soil on infected cucurbit residue.  It may also be seed-borne.  The spores can travel for long distances in moist air currents.

Septoria of Pumpkin
Figure 5. Septoria of Pumpkin

Septoria thrives under cooler, humid conditions of 16- 19°C (61- 66°F).  Initial infection occur in early summer, often with a resurgence when more moderate temperatures return in the fall.

Oedema of Pumpkin
Figure 6. Oedema of Pumpkin

Oedema is often associated with dry conditions during fruit sizing and maturity. The lesions commonly appear where the fruit rests on the soil surface.

For more information on these and other diseases of pumpkins and squash, visit Ontario CropIPM at http://www.omafra.gov.on.ca/IPM/english/cucurbits/diseases-and-disorders/index.html

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