Bacterial blight or Black spot of walnuts

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Journal of the Department of Agriculture, Journal of the Department of Agriculture, Western Australia, Series 4 Western Australia, Series 4

Volume 2

Number 10 October, 1961 Article 12

1-1-1961

Bacterial blight or Black spot of walnuts Bacterial blight or Black spot of walnuts

O M. Goss R. F. Doepel

Follow this and additional works at: https://library.dpird.wa.gov.au/journal_agriculture4 Part of the Horticulture Commons, and the Plant Pathology Commons

Recommended Citation Recommended Citation

Goss, O M. and Doepel, R. F. (1961) "Bacterial blight or Black spot of walnuts," Journal of the Department of Agriculture, Western Australia, Series 4: Vol. 2: No. 10, Article 12.

Available at: https://library.dpird.wa.gov.au/journal_agriculture4/vol2/iss10/12

This article is brought to you for free and open access by the Agriculture at Digital Library. It has been accepted for inclusion in Journal of the Department of Agriculture, Western Australia, Series 4 by an authorized administrator of Digital Library. For more information, please contact [email protected].

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Bacterial blight is the most important disease of walnuts in Western Australia. In bad seasons it can cause heavy losses in susceptible varieties such as Wilson's Wonder, and in some seedling trees.

Carefully timed sprays greatly reduce the incidence of bacterial blight, and it is also recommended that less susceptible varieties—such as Franquette—

should be used for new plantings.

By OLGA M. GOSS, B.Sc. (Hons.), Plant Pathologist, and R. F. DOEPEL, B.Sc. (Agric), Plant Pathologist

B

ACTERIAL BLIGHT is c a u s e d by t h e bacterium X a n t h o m o n a s juglandis which c a n infect leaves, n u t s and also young shoots of t h e c u r r e n t season's growth. T h e disease h a s been recorded from such widely separated districts as Manjimup a n d Bickley Valley.

Survival of t h e bacteria from one season to the n e x t is mainly i n infected d o r m a n t buds. Fallen fruit a n d leaves are also potential sources of disease.

SYMPTOMS On t h e Leaves:

Black a n g u l a r spots, usually surrounded by a lighter halo, appear on the leaves.

(See Fig 1.) On leaves just emerging from the overwintering buds, t h e spots are often restricted to t h e m a r g i n s a n d leaf tips, indicating t h a t infection h a s occurred in t h e d o r m a n t bud during t h e previous growing season.

These m a r g i n a l lesions usually cause considerable leaf distortion as a result of checking of growth in infected areas.

Elongated lesions c a n occur along t h e veins, midrib and t h e leaf stalk.

On t h e N u t s :

On t h e n u t s , t h e disease produces variable symptoms depending on t h e stage of flower or fruit development when i n - fection occurs. In t h e early flowering

Fig. 1—Walnut leaf affected with blight. Note angular blackened spots surrounded by a lighter halo. This leaf also shows the marginal lesions and distortion resulting from early Infections as It emerges from

the bud 833

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B

Fig. 2—Toung walnut Infected during the early flowering stage. Note the blackening at the blossom end. (From

Miller and Bollen)

s t a g e infection results in black spots, m a i n l y a t t h e blossom end. (See Fig. 2.) As t h e n u t develops, these spots m a y en- large, causing a n extensive blackening of t h e tissues. Usually such infected n u t s fall off t h e t r e e long before t h e y reach m a t u r i t y , t h u s considerably reducing yield.

W h e n infection occurs in t h e early post- blossom stage, bacteria frequently gain access t o t h e developing ovary a n d cause a b l a c k e n i n g a n d withering of t h e kernel w h i c h m a y not be visible externally, but r e n d e r s t h e n u t useless. (See Fig. 3.)

L a t e r infections occur mainly on the sides of t h e developing n u t a n d cause depressed, blackened areas r a n g i n g in size from a q u a r t e r of a n inch to over half t h e surface a r e a of t h e n u t . (See Fig. 4.)

T h e s e infections often only invade the fleshy covering a n d do no a c t u a l damage.

I n some cases however, they m a y p e n e t r a t e t o t h e shell a n d cause discoloration with c o n s e q u e n t reduction in m a r k e t value.

(See Fig. 5.)

On t h e Twigs a n d B u d s :

Young shoots of t h e c u r r e n t season, a n d leaf a n d catkin buds a r e also susceptible to a t t a c k . On t h e shoots, depressed lesions develop which m a y encircle t h e stem a n d cause dieback of t h e distal portions.

Diseased buds a r e frequently completely killed, but sometimes t h e disease a t t a c k s only t h e outer scales. This gives rise t o t h e emerging diseased leaves referred to above.

CONDITIONS FAVOURING THE DISEASE AND I T S TRANSMISSION

Development of t h e disease is favoured by cool, moist conditions. T h e occurrence of rainy periods d u r i n g November, Decem- ber or J a n u a r y w h e n t h e n u t s are developing, is likely t o give a b u i l d - u p of t h e disease a n d considerable loss of crop.

During wet w e a t h e r , bacteria ooze from t h e lesions in droplets a n d a r e spread by water splash.

Fig. J— Walnuts infected internally by bacterial blight.

(A) An internally infected n u t as seen from the exterior—the only visible evidence of infection is a slight depres- sion (a) in the tissues at the apex.

(B) A longitudinal median section through an internally infected n u t . Note the extent of in- vasion within. (From Miller and Bollen.)

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Fig. 4—Mature walnuts show- ing symptoms of bacterial blight. Note the depressed black lesions which may ex- tend to almost cover the fruit

CONTROL

1. W h e r e practicable, remove diseased wood from t h e trees—this is more import- a n t in young trees to avoid building u p a source of inoculum.

2. Adopt a routine spray schedule using Bordeaux 5 : 2 : 100 plus l i pints of white oil per 100 gallons as follows:—

(a) Early pre-bloom w h e n 50 per cent, of t h e t e r m i n a l buds have broken.

(b) L a t e pre-bloom—just before most of t h e female flowers come into full bloom.

(c) Early post bloom—when the young n u t s are about 3/16 in. in diameter.

(d) If later r a i n s occur, one or more f u r t h e r sprays m a y be necessary t o keep t h e disease i n check.

The sprays should be applied in mist form w i t h h i g h pressure (400 lb. per square inch is desirable) a n d every effort m a d e to r e a c h t h e tops of t h e trees.

3. Fallen fruit a n d leaves should be raked up a n d disposed of by either plough- ing in or burning, to reduce carryover of t h e disease.

SPRAY EXPERIMENTS

D e p a r t m e n t a l trials using various spray schedules, including streptomycin, have been carried out over a number of years.

Although all sprays gave some measure of control, they proved inferior to t h e above recommendation of Bordeaux 5 : 2 : 100 a n d also to t h e commonly used 6 : 4 : 40 Bordeaux. This l a t t e r spray m a y give better control, but causes some burning of t h e foliage.

REFERENCE

Miller, P. W., a n d Bollen, W. B.—Walnut Bacteriosis a n d I t s Control. Oregon Agri- cultural Experimental Station, Technical Bulletin 9, 1946.

Pig. 5—The effect of bacterial blight on t h e shape and quality of the nuts.

(A) Diseased part of hull adhering tightly to the shell at maturity.

(B) Brown stain left on shell after lesion in the hull is scraped off.

(C) Misshapen n u t caused by the presence of a lesion In the hull.

(From Miller and Bollen.)

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