To determine the effect of moisture and temperature on the growth of mold in harvested walnuts.

Transcription

1 WALNUT MOLDS J E Barton, A R Weinhold, and M N Schroth Temperature Relative Humidity Trials on Harvested Walnuts OBJEJTlVE To determine the effect of moisture and temperature on the growth of mold in harvested walnuts PROCEDURE Field-collected, mature P~e walnuts with average moisture content of 19% were placed into moist chambers with ten different combinations of relative humidity and temperature Temperatures were 15, 21, and 27C and relative humidities were 100, 95, 90, 85, and~ Approximately 80 walnuts were used at each combination RE3ULTS Mold first appeared at low amounts on the second d~ at 27C~, 27C85%, 21C100%, and 21C85% At 15C85% the mold count reached 1~ between 11 and 13 d3 S At 15C~ it took 4 to 5 d~ for the mold to reach 1~ At 15C100%, 21C~, 21C85%, and 27C85%, the proportion of moldy walnuts reached 1~ between d3 3 and d3 4 At the higher regimes, 21C100%, 21C95%, and 27C~, the mold count was 1~ between two and three d3 S At 25C~ the mold count st3 ed below 1~ for over a month (See graphs 1a, b, and c) CONCWSONS There was a two d3 safety period before walnuts began to mold at the higher temperature higher relative humidity regime that was tested Mold developnent was inhibited by low temperature and low humidity and encouraged by high temperature and high humidity This provides the information necessary for determining the best conditions for drying and storing walnuts The Growth of Fungi on Dried and Non-dried OBJEJTlVE Walnuts Dried and non-dried walnuts were placed in a high temperature high humidi ty regime to see if drying has an effect on the amount and timing of mold formation -79-

2 PROCEDURE hulls Half werev dried for 24 hours to about 5% moisture content The walnuts were placed into 1~ RH at Z7 C and observed daily for 10 days RESULTS After 8 days the non-dried hull-non-split kernels were not colonized by fungi The non-dried hull-split walnuts had 15%kernel area covered by mold The dried hull-non-spli t walnuts had 44% mold and the dried hull-split walnuts had C)(:$mold (See graph 2) The dried and non-dried walnuts both had low numbers of moldy walnuts between day 1 and day 3 Between day 3 and day 4 the amount rose to 3% for non-dried and to 61% for dried On the ninth day, the non-dried sample had 53% and the dried sample had 1~ moldy walnuts (See graph 3) CONCLUSONS The dried walnuts exhibited an increased amount of kernel area colonized by fungi After the third day the proportion of moldy walnuts in the dried sample was greater than in the non-dried sample There apparently is a factor in non-dried kernels that inhibits mold formation Variety Trials OBJ]XjTVE Seven varieties from two orchards were tested to determine the amount of kernel and vascular tissue colonization in nuts that mature at different times PROCEDURE Ashley (early harvest), Serr (e~ly harvest), Tehema (middle to late harvest), and Hartley (middle to late harvest) were collected fran Chico four times near harvest Ashley, Vina (early harvest), Chico (early harvest), Earhorn (middle to late harvest) and Hartley were coilected from Brentwood three times near harvest Two samples of thirty walnuts were taken of each variety - one set with split hulls and one set with non-split hulls RESULTS Brentwood: On all dates, the non-split samples had 0 to 10% walnuts colonized with fungi The greatest mold developnent was on walnuts with split hulls and this varied depending on variety and time of harvest On August Z7, Vina (split) had 37% Ashley, Chico, and EBrhorn (split) all had 0% colonized walnuts Hartley (split) was not sampled at this date On September 11, Ashley, Vina, and Chico (split) had between 40 and 47% fungi inside the shell :Farhorn (split) had 30% and Hartley had 10% On October 2, Ashley, Vina, Chico (split) had 1~ of the

3 --- walnuts colonized Earhorn was93% and Hartley was90% (See graphs 5a, b, c, d, and e) Chico: On all dates, only 0 to 3% of the non-split samples were colonized by fungi On September 1, 13% of the Ashley (split) and 7% of the Serr (split) and Tehema (split) were colonized Hartley (split) was not sampled at this date On September 14, Tehema and Ashley (split) were Z7% colonized, Hartley ~ colonized, and Serr 83% colonized On October 1, only Hartley (split) was sampled with 63% colonization (See graphs 6a, b, c, and d) CONCWSONS The non-split walnuts had a very low percentage of kernel or vascular tissue colonized by fungi regardless of time of sample The longer the nuts remained on the tree after hull split, the greater the percentage that became infected Early varieties tended to become colonized faster than later varieties All varieties approached 10Cf1,colonization in the Brentwood orchard by the last sample date Thus, harvesting near hull split increasesthe chances that there is low fungal colonization kernel and hence less chance for high mold counts of the Fungal Colonization of Growing and Maturing Walnuts OlillX::TlVE Further work was done in 1~1 to determine the mode of infection and to pinpoint the infection pathw~ of mold-causing fungi PROCEDURE A spore suspensionof an antibiotic-marked Penicillium (Pe+) was spr~ed onto growing walnuts three times in August and September in Brentwood and Colusa Four samples were taken in August and September RE3ULTS Brentwood : Overall fungal colonizationof the kernel andor vascular tissue increased in non-inoculated walnuts from 7% (Aug 19) to 10% (Sep3) to 67% (Sep 17) (See graph 7) Penicillium accounted for 0%, 3%, and 57% of the moldy nuts (See graph 8) The walnuts with kernel or vascular tissue colonized by Pe+ in the Penicillium-inoculated walnuts went from 17% (Aug 19) to 58% (Sep 3) to 60% (Sep 17) (See graph 9) Unmarked Penicillium accounted for an additional 0%, 6'f" and 23% respectively (See graph 10) Overall hull colonization of non-treated nuts was as follows: Colonization of the stem end went from 2fJ1, (Aug 19) to 40% (Sep 3) to 37% (Sep 17) The blossom end went from 59% to 57% to 33% The hull sections increased from 13% to 13%to 20% (See graph 11) Peniciilium accounted for 0%, af" and 13% of the fungi in the stem end, 0 to 3 to 13% in the blossom end, and 0 to 3 to 15% in the hull sections (See graph 12) Hull colonization by Pe+ with Penicillium-inoculated walnuts -81-

4 was as follows: Colonization of the stem end was 600, 7af" and 67% The blossom end was 83%, ~, and f!:j, The hull sections were 1%, 35%, and 3af, (See graph 13) Unmarked Penicillium accounted for an additional 0, 18, and 1af, on the stem end, 0, 3, and 3% on the blossom end, and 0, 8, and 7% on the hull sections (See graph 14) Colusa: Colonization by any fungus of the kernel or vascular tissue increased in non-treated walnuts from 16% (Aug 20) to 5af, (Sep 3) to 40% (Sep 9) (See graph 15) Penicillium accounted for none of the moldy walnuts (See graph 16) The walnuts with kernel andor vascular tissue colonized by Pe+ in the Penicillium-inoculated walnuts went from o to 27 to 53% (See graph 17) Penicillium accounted for 3 to 0 to 7% of the walnuts (See graph 18) Overall hull colonization of non-treated nuts was as follows: Colonization of the stem end increased from 58% (Aug 20) to 77% (Sep3) to gaf, ( Sep 9) The blossom end went from f37%to 97% to f37% colonized Hull sections went from 17 to 50 to 70% (See graph 19) Penicillium accounted for none of the fungi in any non-treated hull pieces (See graph 20) Hull colonization by Pe+ for Penicillium-inoculated walnuts was as follows: Colonization of the stem end was 13% to 60% to 48% The blossom end went from 47% to gaf, to f37% The hull sections went from 3% to 35% to 48% (See graph 21) Penicillium accounted for none of the stem end or blossom end sections and for 0 to 2 to 2% of the hull sections (See graph 22) CONCDSONS n Brentwood, the amount of Pe+ in the kernel and vascular tissue of the Penicillium-inoculated walnuts was greater than the amount of Penicillium in the non-treated nuts for the pre-harvest samples However, the amounts were the same in the harvest sample Generally, the level of Pe+ in the hull sections of the inoculated walnuts was greater than the Penicillium in the non-treated walnuts The colonization of the kernel and vascular tissue by Pe+ in Colusa increased from 0% in the control to 53% in inoculated walnuts in the harvest sample Hull colonization by Pe+ \<Tasalso increased from af, in controls to 48, f37, and 48% (stem end, blossom end, and hull sections) in inoculated walnuts in the harvest sample Thus we were able to increase the amount of Penicillium in the kernel and vascular tissue by spraying a suspension of spores Wounding was not necessary for colonization Apparently, the blossom end, stem end, and split hull surfaces act as reservoirs to hold the fungi until conditions are right for moving into the shell Penicillium appears to be very important in causing kernel mold Finding the infection pathway requires further statistical analysis The vascular tissue appears to be of primary importance -82-

5 , Graph 1 Fungal colonization of walnuts placed in temperature relative humidity chambers over 45 days a) l5c b) 2lC and c) 27C i x C ) C"') 00 ii X C~ Cltu) eo 4 - ii u) x ClX Clin -Ct CSt e ell ell ell C C 0 C ell ell e ell ell C ell C ell 3 ell G C Z m C C CSt " f) N - Q 01 ), co n f) N - :S 01 eo " SJnN1Yf\ "Olaw " SlnN1Yf\ " " SJnN1Yf\ " " «() r- r- a 0

6 ", 71 a i so i 41 d " UJ NOTP\T - UJ PLT ls "'' "'' Graph 2 Kernel area colonized by fungi on dried and non-dried walnuts : 51 ; 40 -'1 2, DAYS N,RH AT 27 C Graph 3 The proportion of walnuts that visibly moldy in the dried and groups over ten days were non-dried -84-

7 5a) :>D) H18 1', 118 VNA ;a 11 : --! -- ; 48 i, J 18 7 tt OCTa ASHLEY ; ~ lp, 5c) - - e 7 ;a i CHCO ~ j lp" OCT' 5d), 78 J : ; 48-18, EARHORN ~ ~ "'11 5e) HARTLEY 78 t J : ; 48? -, -- 7 lp OCT Graphs 5a, b, C, d, and e Moldy walnuts (%) in Brentwood variety trials: a) Vina b) Ashley c) Chico d) Earhorn e) Hartley -85-

8 "a 6b) on, , i r _-----, 98 SFRR 118 ASHLEY sa i 88 ] 58 :» 58,, i - a -,, 211, MG sa! suo4 OCT SU' sal4 OCT _8 6c) 6d) TEHEHA ' eo Ot till - HARTLEY sa :» -,,,, --,, l i -,, X, rr" r -4' & t & & oj MlG Pt lp! 4 OCT MlGtl Pt Pt4 GeT,,? f>!;)f1s6a, b, c, and d Moldy walnuts (%) in Chico variety trials: a) Serr b) Ashley c) Tehema d) Hartley

9 i - 88 i - 71 ; - ~ 58 ~ 48 :8 ~ : :8 8 MflUTfJ lau«lt 8OT\OD MJ(i4 _'8 SO'4 7 ~ : ~ : ~ 58 z: :8 : ~ :8 8 8 Mflll:ATfJ laulut 8OT\OD i ) MJ(i4 _'8 5fP4 5fP17 ex> J Graph 7 Overall fungi in non-treated walnuts 88 i -, -DlllCtLATfJ laulut i : g : l 58 8OT\OD Graph 8 Non-marked Penicillium in non-treated walnuts ~ ;! ~ 78 u C, -DlllCtLATfJ lau«lts 8tOfT\OOD ~ 48 :8 ~ : :8 8 8 _4 _'8 4 7 Graph 9 Marked Penicillium in Penicillium-inoculated walnuts ~ 48 :8, ~ Z8 :8 8 8 AU84 _'8 SO'4 17 Graph 10 Non-marked Penicillium in Penicillium-inoculated walnuts Graphs 7 to 10 Brentwood inoculations: counts of fungi that colonized the kernel andor vascular tissue ()

10 6 N t 81 w "' 6 r;; T\OOO W: - --,- -_- '(; f -_ trf 4 <0: ill,;, S g \lot ""'\(u,!i i 7,! a 88 u 88 :a l! it 58 i 48 Z8 48 :a, 58 :a -p--=-- AU&4 AUG SEP4 SO'7 AUG4 AUG18 'Ef'4 SO'T 00 Graph 11 Overa 11 fungi in non-treated Graph 12 Unmarked Penicillium in 00 walnuts non-treated walnuts t DOC:tLATfD lau8jts 81 t 81 T\OOO ::=TfD 1lAU8JT1 w "' u M : Go 48 z: e 48 1:1 38 :a 38 Z8 Z8 :a :a AU&4 AU&18 10'4 1U'7 AU84 AU818 1U'4 1U'17 Graph 13 Marked Penicillium in Graph 14 Unmarked Penicillium in Penicillium-inoculated walnuts Penicillium-inoculated walnuts Z8 -'! i Graphs 11 to 14 Brentwood inoculations: counts of fungi that colonized the stem end (5) blossom end (B) and hull sections (H)

11 i - i - LNTUATO ALHJTS CDLUA ; A J: t:i :a :a,, Z Z,, 1 1 A LNTUATO CDLUA lau&lts 1>UG7 AUGZ Kn SU _7 AUOZ Kn 5O'e ]raph 15 Overall fungi in non-treated Grapt: lr Jnrked renici:lium in non-treated 00 walnuts walnuts, \0,_ -DlDClLAT!D ALHJTS d, -NOC\LATfD lau&lts i CDLUA!: CDLUA g ' J: 41 e t:i :a, 51 Z i 28,, 1 E r- - >UG7 AU&Z8 RP3 1>UG7 AUOZ 6D"3 5O'e Graph 17 Marked Penicillium in Graph 18 Unmarked Penicillium in Penicillium-inoculated walnuts Penicillium-inoculated walnuts Graphs 15 to l~ Colusa inoculations: counts of fungi that colonized the kernel andor vascular tissue (r)

12 ,- :! - E COLUSA ffleato laullt! 7 7! ! e 0 J: 48 48!'" t:; '" , '" Z8, 1 AJli7 AUf028 RP3 RJ'8 lajli7 AU021 RP3 SO' Graph 19 Overall fungi 1n non-treated Grapn 20 Unmarked Penicillium in \D 0 walnuts non-treated walnuts! e 88 =: 88! 88, -OC\LATO TS t w ClLUSA 88, -TO laut 01! COLUSA 7 7! 88 l ! 48 J: 48 '" t:; 01, '", 01 i 28 Z8 '" '1 '" ' AJli7 AU028 &U3 iu8 AJli7 AU02, R78 Graph 21 Marked Penicillium in Graph 22 Unmarked Penicillium in Penicillium-inoculated walnuts Penicillium-inoculated walnuts '" Z8 Graphs 19 to 22 Colusa inoculations: counts of fungi that colonized the stem end (8), blossom end (B), and hull sections(h)