Journal of the Department of Agriculture, Journal of the Department of Agriculture, Western Australia, Series 4 Western Australia, Series 4
Volume 5
Number 1 January, 1964 Article 9
1-1-1964
Plant diseases : angular leaf spot of cucurbits Plant diseases : angular leaf spot of cucurbits
O M. Goss
Follow this and additional works at: https://researchlibrary.agric.wa.gov.au/journal_agriculture4 Part of the Bacteriology Commons, Horticulture Commons, and the Plant Pathology Commons
Recommended Citation Recommended Citation
Goss, O M. (1964) "Plant diseases : angular leaf spot of cucurbits," Journal of the Department of Agriculture, Western Australia, Series 4: Vol. 5: No. 1, Article 9.
Available at: https://researchlibrary.agric.wa.gov.au/journal_agriculture4/vol5/iss1/9
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PLANT DISEASES
ANGULAR LEAF SPOT OF CUCURBITS
By OLGA M . GOSS, B.Sc. (Hons.), Plant Pathologist.
A N G U L A R leaf spot caused by the bactterium Pseudomonas lachrymans (E.F.Sm. &
Bryan) Carsner, is common on cucumbers and gherkins grown late in the season. The- same disease has been recorded on rockmelons, watermelons and pumpkins but r a r e l y occurs on these hosts in Western A u s t r a l i a .
Symptoms
The disease appears on leaves, stems and fruit as small water soaked spots. On the leaves they enlarge up to about 3 mms.
in diameter and become angular in shape (Figs. 1 and 2). As they enlarge a tan colour develops on the upper surface and the lower surface appears shiny due largely to the drying out of the bacterial ooze.
On the stems and fruits the spots are covered with a white crusty bacterial exudate (Fig. 3). Inside the fruit a brown
discoloration develops under the spots and continues along the water conducting strands to the seeds, so giving rise to seed infection. (Fig. 4.)
Development of the Disease
The pathogen is seed borne, both i n - ternally and externally and also persists in infected crop residues and in the soil. I t is favoured by the use of fertilisers which are high in nitrogen and low in potassium.
Leaves which are just approaching maxi- mum size are most susceptible to attack and the old mature leaves are most resist- ant. Spread occurs most readily following
p iq ] Early stage of angular leaf spot on rockmelon leaf.
Note angular water-soaked spots. Fig. 2.—Later stage of angular leaf spot on rockmelon leaf.
The spots have enlarged and developed a tan colour.
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Journal of Agriculture Vol 5 No 1 1964
Fig. 3.—Angular leaf spot dis- ease on cucumber fruit. The lesions are covered with a white crusty exudate. (After Ramsay, Weant and L i n k — M a r k e t Dis- eases of Fruit and Vegetables.)
heavy winds accompanied by rain where sprinkler irrigation is used. and Control
The disease is seed borne and careful seed selection and treatment are vitally important.
(1) For seed purposes, carefully select a crop where there is no evidence of the disease.
(2) Treat seed (at least of unknown origin) with mercuric chloride 1-1000 for 5 to 10 minutes, followed by a water rinse. This is only partially effective as it does not kill organisms within the seed, but no method for complete eradication is known at present.
(3) Careful crop rotation and sanita- tion are essential to eliminate the disease.
(4) Do not over manure with nitrogen containing fertilisers but use a fertiliser in which the nitrogen and potash are properly balanced.
(5) If the disease becomes established in a growing crop, copper-contain- ing sprays such as copper oxy- chloride may be used for control.
In young plants it is preferable to use Captan or Ziram for the first sprays and then change to copper when the runners develop. This spray schedule should also help to control powdery mildew disease.
-•gsay*-"1"-
Fig. 4.—Cucumber fruit cut open to show the brownish discolora- tion developed below the spots and continuing along the water- conducting strands to the seeds, giving rise to seed infection.
(After Walker — Diseases of Vegetable Crops.)
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Journal of Agriculture Vol 5 No 1 1964
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Journal of Agriculture Vol 5 No 1 1964
