Downy mildew of cucurbits

Host: Cucumber (Cucumis sativus L.)
Pathogen: Pseudoperonospora cubensis (Berk. & Curt.) Rostov

Distribution

Downy mildew is one of the most devastating diseases of the cucurbits with worldwide distribution. The disease was first reported by Berkeley and Curtis from Cuba in 1868. Since then the disease has been reported in several countries such as, Americas, Europe, Japan, Australia, South Africa, Middle east and Indian subcontinent. The disease becomes destructive in warm and humid climate, which provide ideal conditions for the fungal pathogen.

Symptoms

Downy mildew of cucurbit
Foliage of cucumber infected with Downy mildew
Downy mildew of cucurbits Downy mildew of cucurbits
Angular lesions caused by Downy mildew Leaf necrosis caused by Downy mildew

Symptoms of the disease are confined to the leaves, where these appear in form of angular lesions. Under favorable conditions (i.e., dew or irrigation) lesions become water-soaked, which later on become chlorotic and nectrotic. At this stage plant cell dies and curled-up. This stage is marked as ‘wildfire’ because of burned appearance of foliage. Although, the fructifications are not infected, however, heavy loss in yield occur because of reduction in photosynthetic area of leaves.


Pathogen

Downy mildew of cucurbits is cause by Oomycetous fungus, Pseudoperonospora cubensis. It is an obligate parasite or biotrophs. Fungus produces singly-borne sporangiophores, which terminates into pointed tips at acute angles. Large conspicuous lemon-shaped sporangia born on the tip. The sporangium has an apical papilla. Surface of sporangium is finely decorated. Sexual reproduction in this fungus is rare (Colucci et al., 2010).

Control measures

  • Reduced humidity and proper airflow between the leaves reduce the disease severity.
  • Pathogen very rapidly acquire resistant towards the fungicides. Hence, multiple fungicides on rotation basis should be applied under controlled environmental conditions, possibly by integrating with other measures.
  • Wild cucumber (Cucumis metuliferus L.) and IIHR-438 genotype of cucumber have shown to be resistant against downy milder in tropical climate of India (Bommesh et al., 2018).

References

  • Bommesh, J.C., Pitchaimuthu, M., Sadashiva, A.T., Sriram, S., Varalakshmi, B. and Ravishankar, K.V., 2018. Identification and confirmation of downy mildew (Pseudoperonospora cubensis Berk. & Curt.) resistance sources in cucumber (Cucumis sativus L.). Indian Phytopathology 71(3), pp.337-348.
  • Colucci, S.J. and Holmes, G.J., 2010. Downy mildew of cucurbits. Downy mildew of cucurbits. DOI: 10.1094/PHI-I-2010-0825-01


Powdery mildew of cucurbits

Host: All the plants of family Cucurbitaceae
Pathogen: Podosphaera xanthii (syn. Sphaerotheca fuliginea) and Golovinomyce cucurbitacearum (syn. Erysiphe cichoracearum)

Distribution

Powdery mildew of cucurbits is distributed throughout the world, where ever cucumbers, watermelons, pumpkins, squashes and gourds are grown. The disease has been extensively studied in the Panjab state of Northern India by Gupta and Sharma (2012) and from Florida by Nuñez-Palenius et al. (2006).

Symptoms

Powdery mildew Powdery mildew of cucurbits
Powdery mildew affected cucumber leafPowdery mildew on upper surface of infected leaf
Powdery mildew of cucurbits Powdery mildew of cucurbits
Powdery mildew on lower surface of infected leafPowdery mildew on infected stem

The disease is very easy to identify as the pathogen covers the leaves and stems in the form of white pad of mycelium. Powdery mildew appears first on older stem and leaves of cucurbits. White powdery mass of fungal mycelium appears on both leaf surfaces, stems, and petioles. In severe infection, the disease can cause premature death of leaves and reduce yield and fruit quality. Premature death of plant can also occur.


Pathogen

More than one fungal pathogens are reported to incite the disease. Gupta and Sharma (2012) reported that Podosphaera xanthii (syn. Sphaerotheca fuliginea) and Golovinomyces cichoracearum (syn. Erysiphe cichoracearum) are responsible of causing powdery mildews in cultivated and on wild Coccinia cordifolia respectively in Punjab of Northern India.

Control measures

  • Spray of Sulfur and Copper based fungicides prevent plants from powdery mildew. The disease once is established, it cannot be controlled through the fungicides.
  • Resistant cultivars of cucurbits should be grown.
  • Plantation of cucurbits in areas with proper aeration is helpful. Shade favors the disease.
  • Avoid over dose of nitrogenous fertilizers.

References

  • Gupta, M.K. and Sharma, G.K., 2012. Species composition of powdery mildew on cucurbits in Punjab, India. Asian Journal of Microbial. Biotech. & Env. Sc, 14, pp.257-262.
  • Nuñez-Palenius, H.G., Hopkins, D. and Cantliffe, D.J., 2006. Powdery mildew of cucurbits in Florida. IFAS Extensions-University of Florida, pp.1-9.